Update tag for stable version

[louispt1]

Bug Description

At the moment the tag for the stable version is stable.2025.01. We ought to change it to 2025.S1, and update future tags to match this new simpler format.

This issue is to update the tag to match this new format. That may require making a new tag on the same commit, and then deleting the old one. We will also need to update the docs, and anywhere else this tag is referenced

Where to make changes

An inventory of the places this tag would need to be changed:

• Server: This is the most difficult change, but could also present an opportunity to clean up any unnecessary volumes left over from testing / previous deployments. We would have to:
  • Redeploy the 8 containers that constitute the stable engine/model instances. (redis, memcached, web, sidekiq)
  • Update the relevant Ansible variables: stable_instance_2025_01_group, VERSION_TAG: 'stable.01', ETSOURCE_REF: '2025-01' etc - this is not so quick because we have to be quite careful so nothing gets lost in the change.
• Update the deploy_stable .semaphore configurations in Engine/model
• Tag Sentry releases appropriately
• Update the tags on Engine/Model GH to point to the correct commits
• Update docs to name the releases correctly in Releases page and previous commit references

Next steps:

• Explicitly agree on the format of stable version and the 'regular' releases
  I think we agreed to list the regular four-weekly releases numerically: e.g. 2025.01, 2025.02 and the stable releases like: 2025.S1, but before preparing this work I'd like to double check that we still agree on this.
• Refine the changes needed in each repository into individual issues - this issue can serve as a parent issue.

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