feat: use pebblegozstd tags everywhere

[kruskall]

Motivation/summary

The apm-server binary is built as a static Go binary with CGO disabled, so it relies on the pure Go zstd implementation. However, our test suite runs with CGO enabled (required for the race detector), which means it previously exercised a different code path.

To align the test builds with the production binary, we now compile the tests with the pebblegozstd build tag. This forces Pebble to use the Go ZSTD library in both environments.

The same tag also affects the FIPS build, which previously depended on the C-based zstd library due to Microsoft’s Go fork requiring CGO. Switching to the Go implementation brings the FIPS build in line with the standard binary and enhances security by eliminating the C-based zstd dependency.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

make test

Related issues

Closes #19626

[github-actions]

🤖 GitHub comments

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• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label. Could you fix it @kruskall? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

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