tests: Remove old system for checking for regressions

[sentrivana]

Description

With all eligible test suites now moved to toxgen, we can remove our old way of testing for regressions in new versions of packages.

For context, before toxgen we had two groups of test targets for each test suite: pinned and latest. Pinned targets used to be a small selection of hand-picked pinned versions of the package to test. Latest always pulled the newest release, to check for incompatibilities. There was a lot of code to cater to having these two different categories.

With the switch to toxgen, all targets are now pinned, and we make sure we pick a representative sample of versions to test against, always including the latest available version to detect regressions. I'm removing the code dealing with the latest group entirely, and removing references to anything pinned as this is now simply the default.

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[sl0thentr0py]

think someone with gh admin will need to update the checks?

[sentrivana]

think someone with gh admin will need to update the checks?

Yeah will bother Johannes with this on Mon