Automatically bump k3s latest version in watch-dependencies

[manics]

Proposed change

In #1684 I replaced the K3s latest version with 1.26 to reduce the time spent investigating failures related to new versions of k3s. If we automatically update the k3s version in the watch-dependencies workflow we could go further and pin the full 1.26.x version.

Alternative options

Bump the latest k3s version manually

Who would use this feature?

CI maintainers

(Optional): Suggest a solution

Since it's not possible to use a workflow environment variable in a matrix the latest k3s version needs to be updated in multiple places. It might be helpful to add a marker comment to make parsing easier, e.g.
- v1.26 # <<k3s-latest-managed-by-watch-dependencies

[consideRatio]

I think I'd rather have stable k3s version referenced than having:

• a hardcoded version bumped manually (k3s 1.26 as now)
• adding and maintaining automation to bump the hardcoded version

Before we had both latest and stable, but I think we should stick to stable because then we avoid an intermittent k3s related failure where their release artifacts aren't yet published.

The most recent failure was a failire in binderhub depending on z2jh 2, that isn't compatible with k8s 1.27 - because z2jh 2's user-scheduler isn't compatible with k8s 1.27.