Handle system index set scoping

[patrickmann]

Resolves Graylog2/graylog-plugin-enterprise#5904

#21148 introduced scoping for index sets. We now need to make sure scope is accounted for.

Description

• ensure that all system index sets have scope set to SystemIndexSetScope
• use scope-aware methods for write operations on index sets

Motivation and Context

See linked issue

How Has This Been Tested?

• first migration runs successfully
• multiple migrations
• system index sets cannot be deleted
• regular index sets can be deleted

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 updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.