Drop support for Active Record 7.2

[nertzy]

Summary

• Raise the minimum supported Active Record version to 8.0 now that Active Record 7.2 is end of life.
• Remove the Active Record 7.2 release and stable-branch jobs from CI.
• Remove obsolete table-existence, dependency-cache, and redundant Active Support descendants-cleanup paths.
• Retain WithModel's own descendants filtering because Active Record 8 still exposes destroyed temporary classes when descendant clearing is disabled.
• Follow Rails main by prepending the filter so later class-method extensions cannot bypass it, while keeping the patch scoped to Active Record.

Test plan

• Run the full suite against Active Record 8.0 (133 RSpec examples and 10 minitest runs).
• Run the full suite against Active Record 8.1 (133 RSpec examples and 10 minitest runs).
• Reproduce descendants behavior with clearing disabled under both supported Active Record versions.
• Verify a later Active Record class-method extension cannot bypass filtering.
• Run Standard and diagnostics on the compatibility-cleanup files.
• Verify the gemspec requires Active Record 8.0 or later.
• Build the gem successfully.

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