v8.5.0

Changed

• Bumped the gitsemver CLI in the image from v2.0.1 to v3.0.1. v3 generates the new dev version schema
  X.Y.Z-r<branch-hash>t<timestamp>h<sha>, which never produces an invalid Kubernetes label value.
• The gitsemver install step downloads the raw gitsemver-linux-<arch> binary instead of a .tar.gz
  archive. gitsemver stopped publishing those archives in
  gitsemver#260, so the old URL 404s on v3.
• Bumped the gitsemver Go library from v2.0.1 to v3.0.1. The import path moves to
  github.com/giantswarm/gitsemver/v3. Only TopLevel, Config, New, IsValidStable and
  ReferenceNotFoundError are used here, and none of them changed. The v3 removals
  (Config.MaxVersionLength, GS_MAX_VERSION_LENGTH) are not referenced.
• The go directive moves from 1.25.0 to 1.26.0, because gitsemver v3 declares it. CI and the image already
  build with Go 1.26.
• The pre-commit CI gate now installs golangci-lint 2.13.2 on Go 1.27, which is what the central
  giantswarm/github template pins. golangci-lint 2.9.0 is built with go1.26 and refuses a module that
  targets go1.27, so the gate would have failed on every pull request as soon as the toolchain directive
  moved. opsctl hit exactly that in
  opsctl#2684.