Sync Go version checks across CI, mise.toml, and docs

Summary

Renovate bumped GO_VERSION in .gitlab/ci/_common.gitlab-ci.yml to 1.26.5 via !7024 (merged), but nothing kept mise.toml's golang version or the Go version strings in docs/development/_index.md in sync. The docs mismatch only surfaced later as a QA job failure on an unrelated MR (!6928 (merged)), because the docs:check development docs Go version job only ran on MRs that touched docs/**, not on the MR that actually changed GO_VERSION. mise.toml had drifted silently with no check at all.

This MR:

• Fixes the current drift in mise.toml and docs/development/_index.md.
• Broadens the docs Go version check's trigger to also fire on changes to .gitlab/ci/_common.gitlab-ci.yml or mise.toml, so a future mismatch fails fast on the version-bump MR itself.
• Adds a matching check mise.toml Go version job.

Related: !7068 (merged) also fixes the docs content on its own; this MR includes the same docs fix so its own pipeline passes the new gate.

Test plan

• docs:check development docs Go version passes on this MR.
• New check mise.toml Go version job passes on this MR.
• Confirm prettier formatting of .gitlab-ci.yml files is unchanged (npx prettier --check ".gitlab/ci/*.{yaml,yml}" passes locally).

Closes #39655 (closed)