[DOCS] Remove links to master

[lcawl]

Description

Relates to elastic/docs#3160

This PR fixes the following broken link:

INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/serverless/current/observability-case-settings.html contains broken links to:
--
  | INFO:build_docs:   - en/kibana/master/thehive-action-type.html

I think this PR only needs to be merged if we assume that we'll want to stop the main branch of the Kibana Guide before we stop building the only branch of the Serverless Guide.

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

Closes #

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]

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[eedugon]

LGTM