[DOCS] Fix hard-coded links to master

[lcawl]

Description

Relates to elastic/docs#3160

This PR fixes the following type of documentation build error, which occurs when we retire the master documentation:

INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/observability/8.6/whats-new.html contains broken links to:
--
  | INFO:build_docs:   - en/kibana/master/opsgenie-action-type.html

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Relates to elastic/docs#3160

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

[github-actions]

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[github-actions]

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[kilfoyle]

LGTM! 🦞