chore: remove legacy backends

[LegendPei]

Purpose of the PR

• close [Breaking Change] Remove unnecessary backend and all related things #2748

Remove the backend implementations that have been unsupported since 1.7.0, and
clean up their related build, distribution, test, release, and documentation
artifacts.

Main Changes

• Remove Cassandra, ScyllaDB, MySQL, PostgreSQL, and Palo backend modules.
• Remove related Maven dependencies, distribution JAR entries, Docker/CI scripts,
  test cases, backend-specific runtime branches, and logging configuration.
• Clean obsolete release dependencies, LICENSE/NOTICE entries, and license files.
• Remove Cassandra-specific initialization retry handling and use generic backend
  initialization retries instead.
• Update README and HBase runbook:
  • 1.7.0+ supports RocksDB, HStore, HBase, and Memory only.
  • Historical versions earlier than 1.7.0 supported MySQL, PostgreSQL,
    Cassandra, ScyllaDB, and Palo.

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • xxx

Does this PR potentially affect the following parts?

• Dependencies (add/update license info & regenerate_known_dependencies.sh)
• Modify configurations
• The public API
• Other affects (distribution packaging, CI scripts, and backend documentation)
• Nope

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need

[LegendPei]

@imbajin Hello, this pull request (PR) involves significant changes. I'm not sure if I misunderstood the meaning of this issue and made some incorrect deletions or modifications. Should I delete the old backend code such as MySQL, or just remove dependencies or registry center-related code?

[imbajin]

@imbajin Hello, this pull request (PR) involves significant changes. I'm not sure if I misunderstood the meaning of this issue and made some incorrect deletions or modifications. Should I delete the old backend code such as MySQL, or just remove dependencies or registry center-related code?

Your understanding is correct. Our main goal is to gradually remove these unmaintained backends, along with all related code and LICENSE files, to keep the repository clean.

You're also welcome to join our organization to connect and learn—just send me an email so I can invite you!

[imbajin]

The legacy backend removal is directionally correct, but the distribution and release inventories still need cleanup before merge.

🧹 Minor: Please also remove the obsolete root-POM RAT exclusion **/apache-cassandra-*/**. Its corresponding generated test data/script has been removed, so retaining the exclusion could allow future matching files to bypass license checks.

[imbajin]

The previous functional, packaging, dependency, and license findings are resolved. This follow-up contains two remaining behavior/test issues plus repository-context cleanup; HBase is intentionally retained as deprecated pending its planned 2.0 removal.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.16%. Comparing base (c10779d) to head (fa65e25).

Files with missing lines	Patch %	Lines
...apache/hugegraph/security/HugeSecurityManager.java	0.00%	6 Missing ⚠️
.../main/java/org/apache/hugegraph/cmd/InitStore.java	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (c10779d) and HEAD (fa65e25). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (c10779d)	HEAD (fa65e25)
	4	1

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3116      +/-   ##
============================================
- Coverage     37.48%   32.16%   -5.33%     
+ Complexity      520      264     -256     
============================================
  Files           808      770      -38     
  Lines         69594    65772    -3822     
  Branches       9169     8725     -444     
============================================
- Hits          26089    21153    -4936     
- Misses        40720    42172    +1452     
+ Partials       2785     2447     -338     

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[imbajin]

Follow-up review on the updated cleanup. The runtime, build-profile, dependency-inventory, and previously reported test issues are resolved. One additional low-priority documentation cleanup remains outside the changed lines: 🧹 hugegraph-server/README.md:11 still ends its built-in backend list with /..., which suggests additional built-in implementations remain after this removal. Please list the remaining implementations explicitly; custom backend extensibility is already described by the rest of the sentence.