Cloudwatch: Update cloudwatchNewRegionsHandler to General Availability.

[sarahzinger]

What is this feature?

Should involve no user facing changes, this is simply moving a feature toggle to general availability. The feature toggle controls a sizable refactor of our logic for fetching regions for the cloudwatch plugin api. We had put it behind a feature toggle to ensure we could release the refactor with a good plan to revert in case of unexpected bugs.

Relates to:

• https://github.com/grafana/oss-plugin-partnerships/issues/293
• Original pr where we introduced this feature toggle: Cloudwatch: Fix Unexpected error #74420

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.