[2.16] Revert change to use System Index Registry from core to determine if request matches system indices

[cwperks]

Description

Preparing this revert PR against 2.16 in case this change needs to be reverted.

During 2.16.0 release prep, integ tests with security were seen failing in Security Analytics Plugin and ISM:

• Security Analytics: [BUG] All system index interactions should happen in a stashed context security-analytics#1182
• ISM: [AUTOCUT] Integration Test failed for index-management: 2.16.0 index-management#1212

Both of these plugins have SystemIndices registered through the extension point SystemIndexRegistry.getSystemIndexDescriptors, but do not have corresponding entries tracked in security plugins list of system indices. (See #4439 (comment) for registered system indices vs list tracked by security plugin)

Each plugin needs to refactor to ensure that system index interaction is done in a privileged block, but I'm raising this PR in case a revert is needed.

There are 3 possible ways to move forward:

1. Revert this change and only use the list of system indices tracked by the security plugin here
2. Temporarily remove SytemIndexRegistration via SystemIndexPlugin.getSystemIndexDescriptors for affected plugins
3. Wrap system index interaction in affected plugins to ensure that plugins interact with system indices in a privileged block.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.42%. Comparing base (deef041) to head (4e8fba4).

Additional details and impacted files

@@            Coverage Diff             @@
##             2.16    #4616      +/-   ##
==========================================
- Coverage   65.42%   65.42%   -0.01%     
==========================================
  Files         316      316              
  Lines       22103    22102       -1     
  Branches     3572     3572              
==========================================
- Hits        14461    14460       -1     
- Misses       5851     5852       +1     
+ Partials     1791     1790       -1     

Files	Coverage Δ
...earch/security/privileges/PrivilegesEvaluator.java	71.87% <100.00%> (ø)
...urity/privileges/SecurityIndexAccessEvaluator.java	70.40% <100.00%> (ø)

... and 2 files with indirect coverage changes