Fix release/v1.12 branch

[kron4eg]

What this PR does / why we need it:
This PR fixes all the linter issues and moves release/v1.12 to github actions too, like release/v1.13

What type of PR is this?
/kind failing-test

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

* upgrade to go 1.26
* fix vulnerable golang.org/x/net
* fix vulnerable github.com/containerd/containerd

Documentation:

NONE

[kubermatic-bot]

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[kron4eg]

/retest