[chore] remove stable requirement from enum rule

[trisch-me]

Fixes #1686

before the fix with missing enum attribute check policies don't have any error

After the fix

semantic-conventions[main*] % make check-policies
Weaver Registry Check
✔ `main` semconv registry `/home/weaver/source` loaded (172 files)
✔ `baseline` semconv registry `https://github.com/open-telemetry/semantic-conventions/archive/refs/tags/v1.29.0.zip[model]` loaded (171 files)
✔ No `before_resolution` policy violation
✔ `main` semconv registry resolved
✔ No `after_resolution` policy violation
✔ `baseline` semconv registry resolved
✔ All `comparison_after_resolution` policies checked (1 violations found)

Diagnostic report:

Violation: Enum 'gen_ai.system' had member 'vertex_ai', but is no longer defined
  - Category         : backward_compatibility
  - Type             : semconv_attribute
  - SemConv group    : registry.gen_ai
  - SemConv attribute: gen_ai.system
  - Provenance: /home/weaver/source

after adding a deprecated attribute

semantic-conventions[main*] % make check-policies
Weaver Registry Check
✔ `main` semconv registry `/home/weaver/source` loaded (172 files)
✔ `baseline` semconv registry `https://github.com/open-telemetry/semantic-conventions/archive/refs/tags/v1.29.0.zip[model]` loaded (171 files)
✔ No `before_resolution` policy violation
✔ `main` semconv registry resolved
✔ No `after_resolution` policy violation
✔ `baseline` semconv registry resolved
✔ No `comparison_after_resolution` policy violation

[jsuereth]

Looks good, but would you mind also updating the test here:

https://github.com/open-telemetry/semantic-conventions/blob/main/policies_test/compatibility_test.rego#L275

[trisch-me]

@jsuereth @lmolkova I have extended the test for experimental attributes