chore(deps): update dependency golang/go to v1.27.0

This MR contains the following updates:

Package	Update	Change
golang/go (source)	minor	1.26.7 → 1.27.0

Merge order with !2020 (merged)

Merge this MR before !2020 (merged). This MR adds a sentence to guardrail 7 in AGENTS.md recording that standalone thelper v0.7.1 aborts on a tree pinned to Go 1.27, and !2020 (merged) moves that whole block out of AGENTS.md into docs/dev/go-testing.md. If !2020 (merged) lands first the new sentence is dropped with the block and nothing reports it; if it lands second it conflicts, which is visible and fixable. Whoever rebases !2020 (merged) needs to carry the Go 1.27 sentence into docs/dev/go-testing.md.

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Changes on top of the Renovate bump

• Adapted the tree to the Go 1.27 toolchain: encoding/json/v2 version gating, embedded struct literals flattened for the new modernize checks.
• Corrected the go directive forward-compatibility claim in the docs.
• Resolved the .gitlab-ci-asdf-versions.yml conflict with main.