Point to a UKCA tag instead of a commit hash

[Whyborn]

Points to the UKCA tag which is the current head of the um13.1_blackcarbon_cable_deposition branch, which contains the changes required for CABLE. There is no functional change- the 2026.03.0 tag points to the same commit as is used currently.

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🚀 The latest prerelease access-am3/pr33-1 at 7e36627 is here: #33 (comment) 🚀

[github-actions]

The model version in the spack.yaml has not been updated.
You can update it manually.
Alternatively, comment the following to have it updated and committed automatically:

• !bump major for feature releases
• !bump minor for bugfixes

[github-actions]

🚀 Attempted to deploy access-am3 Prerelease pr33-1 with commit 7e36627

🖥️ Gadi Deployment ❌

[Whyborn]

Thoughts on upping the version- there is no functional change here. The UKCA change is also dated based on when the tag was created, rather than when the last modification was, so it's currently at a "higher" dated version than AM3 itself, which would be a bit strange. Thoughts @bschroeter? Might just be easier to up the AM3 version

[Whyborn]

Actually, I have this PR which is ready to go- confirmed bitwise compatibility between builds using this configuration. I can just add the UKCA reference change there?

[MartinDix]

Should the tag be called am3_2026.03.0? Just the date seems too much like a release version rather than a branch.

[Whyborn]

Fair point- especially if CM3/rAM3 may be using the main branch for some time before CABLE is included. I'll make that change.

[Whyborn]

I've decided to merge this change into this PR for simplicity. This is now superseded.