Fix a problem with a random rotation strategy order

[luk-kaminski]

Description

Fixes #12246
Rotation strategies are returned in the same order in /system/indices/rotation/strategies endpoint.
TimeBasedRotationStrategy is the first/default one if nothing is configured in graylog.conf file.
Validator for "enabled_index_rotation_strategies" will not allow to put duplicates in this property.

Motivation and Context

Every time the Create Index Set page was opened, the Index Rotation Configuration gets randomly selected from provided 3 options. It was caused by the fact that back-end returned the strategies in random order.

How Has This Been Tested?

1. Open the System -> Indices page
2. Click on the Create Index Set button
3. Verify which rotation strategy is selected
4. Refresh the page and verify again what strategy is pre-selected now

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

[todvora]

Thanks, looks perfect!