Register the snapshot_archiving feature flag

[fpetersen-gl]

Description

Adds one line to feature-flag.config: snapshot_archiving=off.

Motivation and Context

Feature flags are registered only in graylog2-server, so the entry for the enterprise Snapshot Archiving work belongs here. The plugin reads the flag to hide the Snapshot Archiving settings and to refuse them at the API.

Graylog treats an unknown flag as off, so this can merge before or after the plugin side without changing behaviour either way.

Part of Graylog2/graylog-plugin-enterprise#14686.
/nocl - Part of something bigger.

How Has This Been Tested?

ImmutableFeatureFlagsTest passes (12 tests). The line itself carries no logic, so it has no test of its own.

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.