chore: let Renovate track Go minor version in setup-go workflows

[mstoykov]

Summary

Adds a Renovate customManager so the Go version pinned in our GitHub Actions workflows gets bumped automatically when a new Go minor release ships, instead of needing manual PRs.

The regex matches two forms used across the workflows:

• Plain: go-version: 1.26.x
• Matrix: go-version: [1.26.x]

Only the major.minor portion is replaced — the .x suffix stays, so workflows still pick up the latest patch automatically.

Files covered

• lint.yml, tc39.yml, wpt.yml, xk6.yml
• test.yml, test-browser.yml (both [1.25.x] and [1.26.x] matrix entries)

Intentionally excluded

• browser_e2e.yml — uses 1.x (floating major, doesn't need bumping)
• build.yml — Go version comes from a workflow input, not a literal

Notes

• The two [1.25.x] matrix entries in test.yml / test-browser.yml look like deliberate N-1 lagging. With this manager, Renovate will propose bumping them too — if we want them pegged behind, a follow-up packageRule with matchFileNames + allowedVersions can constrain them.
• Renovate needs a GitHub token at runtime to resolve github-releases lookups (already the case in CI).

Test plan

• renovate-config-validator passes on the new config
• Regex matches exactly the 8 expected lines (verified via grep)
• Local renovate --dry-run=lookup extracts depName: golang/go for each matched file; with one file temporarily downgraded to 1.20.x, Renovate flags it as a separate update candidate from the others on 1.26.x
• After merge: confirm Renovate opens the expected PR(s) on the next scheduled run