wip: make GZIP GA and enabled by default

[AndersonQ]

Proposed commit message

filebeat: make GZIP GA and enabled by default

This commit refactors the GZIP support in the filestream from beta to GA and enables it by default.

The `GZIPExperimental` config field has been replaced with `GZIPDisabled` to make the zero value (`false`) represent the new default behavior (GZIP enabled). The logic throughout the input has been inverted to reflect this change.

For backward compatibility, `GZIPExperimental` is retained in the config struct as a `*bool`. This allows detecting if the field was explicitly set by the user. If it is set, its value is ignored, and a deprecation warning is logged, guiding the user to the new `gzip_disabled` flag.

The validation logic in `config.Validate()` is updated to check `!c.GZIPDisabled`. The error message is now more explicit, instructing developers to set `gzip_disabled: true` if a `file_identity` other than `fingerprint` is required for their use case.

Unit and integration tests have been updated accordingly to remove dependencies on the old `gzip_experimental` flag and to verify the new default behavior, the disable flag, and the deprecation warning.

AI tools used: Cursor.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

Ingesting GZIP files is enabled by default. It conflicts with file identities other than fingerprint. If another file identity is used, and error is logged and the input does not start. The error guides the user to disable GZIP ingestion.

A new configu

Author's Checklist

• [ ]

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This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @AndersonQ? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
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• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
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