[9.1](backport #47763) feat: switch back to upstream pkcs8 lib

[mergify]

Proposed commit message

replace elastic fork with upstream pkcs8 lib

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

Author's Checklist

• [ ]

How to test this PR locally

Related issues

Followup to elastic/elastic-agent-libs#344

Use cases

Screenshots

Logs

---

This is an automatic backport of pull request #47763 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 908d48f has failed:

On branch mergify/bp/9.1/pr-47763
Your branch is up to date with 'origin/9.1'.

You are currently cherry-picking commit 908d48fcb.
  (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:   NOTICE.txt
	modified:   go.sum

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   go.mod
	deleted by us:   libbeat/otelbeat/oteltest/tls_helper.go

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

[botelastic]

This pull request doesn't have a Team:<team> label.

[github-actions]

🤖 GitHub comments

Just comment with:

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

[kruskall]

does not use pkcs8 directly