chore: drop support for Bazel 6.4.0 when consuming from source

[mattem]

Drops testing (and therefore support) for consuming rules_py from source on Bazel 6.

---

Changes are visible to end-users: no

Test plan

• Covered by existing test cases

[mattem]

• #418
• #417
• #420 👈
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

Join @mattem and the rest of your teammates on Graphite