Add deprecation warning for merging SARIF files with non-unique categories

[koesie10]

This adds a deprecation warning for merging SARIF files with non-unique categories. This is behind a feature flag.

These are the criteria for showing the deprecation warning:

• The feature flag is enabled
• If on GHES: GHES version >= 3.14.0
• The deprecation warning hasn't been shown in the same run
• Non-unique categories are found

Merge / deployment checklist

• [ ] Confirm this change is backwards compatible with existing workflows.
• [ ] Confirm the readme has been updated if necessary.
• [ ] Confirm the changelog has been updated if necessary.