Offer functions for outdated indices

[moesterheld]

Description

This PR bundles the changes needed for handling outdated indices. It is based on #26076 and #25860 . Please merge these first and then rebase this branch on master.

It adds:

• a delete endpoint for indices that are outdated and non-Graylog managed (including validation)
• validation that indices to be reindexed are system indices
• usage of the admin cert based authentication OS client in the OutdatedIndexService for all calls

Motivation and Context

part of https://github.com/Graylog2/graylog-plugin-enterprise/issues/14002

How Has This Been Tested?

existing tests, manually

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.