Update Gradle wrapper to 8.7

[infotexture]

Description

Update Gradle distribution version & regenerate wrapper to pick up recent changes per https://docs.gradle.org/current/userguide/gradle_wrapper.html#sec:upgrading_wrapper

./gradlew wrapper --gradle-version 8.7
./gradlew wrapper --gradle-version 8.7

https://docs.gradle.org/8.7/release-notes.html

Motivation and Context

Follow-up test to #554 as complement to dita-ot/dita-ot#4447

How Has This Been Tested?

• Created fresh worktree in core repo
• Updated Gradle wrappers in both core and docs repos to version 8.7
• Used latest Gradle refactoring from Update Gradle to 8.7 dita-ot#4447
• Tested with Java 17 and Java 21

Results

• ❌ Java 17 build fails
• ❌ Java 21 build fails

Note
To test, the wrappers must be updated in both core & docs repos.
See corresponding PR in core repo: dita-ot/dita-ot#4447.

[infotexture]

@jelovirt A few things left to try here:

• It might make sense to run the same IntelliJ refactoring on the docs Gradle build here in this branch and update the docsrc submodule to point to the result to see if that changes anything.

• In the docs branches, I re-ran the Gradle wrapper task twice per Gradle recommendations:

  If you want all the wrapper files to be completely up-to-date, you will need to run the wrapper task a second time.

  Might help to do the same in Update Gradle to 8.7 dita-ot#4447.