[9.1] (backport #18121) feat: bump go to 1.25.1

[mergify]

Motivation/summary

bump to latest stable version

go 1.25 adds native auto maxprocs so we don't need the automaxprocs lib
anymore.

remove dependency and related tests

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

Related issues

Blocked until wolfi go 1.25.0 image is published

---

This is an automatic backport of pull request #18121 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 632899a has failed:

On branch mergify/bp/9.1/pr-18121
Your branch is up to date with 'origin/9.1'.

You are currently cherry-picking commit 632899af.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .github/workflows/ci.yml
	modified:   .go-version
	modified:   NOTICE-fips.txt
	modified:   NOTICE.txt
	modified:   cmd/intake-receiver/go.mod
	modified:   integrationservertest/go.mod
	modified:   internal/beatcmd/beat.go
	modified:   internal/beatcmd/beat_test.go
	deleted:    internal/beatcmd/maxprocs.go
	deleted:    internal/beatcmd/maxprocs_test.go
	modified:   systemtest/apmservertest/server.go
	modified:   systemtest/go.mod

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   go.mod
	both modified:   go.sum

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

🤖 GitHub comments

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Just comment with:

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

[v1v]

I manually triggered the DRA packaging:

• https://buildkite.com/elastic/apm-server-package/builds/6290

As long as #18581 is merged we won't need to manually trigger those builds.

[v1v]

@kruskall , there are failures with the packaging:

2025-09-24 14:52:48 CEST | 3 warnings found (use docker --debug to expand):
-- | --
  | 2025-09-24 14:52:48 CEST | - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 8)
  | 2025-09-24 14:52:48 CEST | - InvalidDefaultArgInFrom: Default value for ARG [REDACTED]/wolfi/go-msft-fips:${GOLANG_VERSION} results in empty or invalid base image name (line 8)
  | 2025-09-24 14:52:48 CEST | - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 67)
  | 2025-09-24 14:52:48 CEST | Dockerfile.fips:19
  | 2025-09-24 14:52:48 CEST | --------------------
  | 2025-09-24 14:52:48 CEST | 18 \|
  | 2025-09-24 14:52:48 CEST | 19 \| >>> RUN --mount=type=cache,target=/root/go/pkg/mod \
  | 2025-09-24 14:52:48 CEST | 20 \| >>>     --mount=type=cache,target=/root/.cache/go-build \
  | 2025-09-24 14:52:48 CEST | 21 \| >>>     make apm-server-fips-msft
  | 2025-09-24 14:52:48 CEST | 22 \|