Add renovate dependency updates (#2027)

[shivakunv]

(cherry picked from commit c8c73ff)

TL;DR

Add a self-hosted Renovate workflow to automate Go module dependency updates
in this repository. Today there is no dependabot.yml or renovate.json at
the repo root, and no bot has ever opened a PR here. Chart and image version
pins already get bumped automatically by chart-version-bump.yml and
stack-pin-bump.yml, but Go module dependencies (go.mod/go.sum) across
the roughly 35 native Go subtrees under src/, tools/, tests/, and
examples/ are manual .

For the Reviewer

• .github/renovate.json added, scoped to enabledManagers: ["gomod"],
  targeting main, with non-major Go dependency bumps grouped into one PR
  and major bumps opened individually.

• ignorePaths excludes vendor/**, the two upstream-owned subtrees
  (ess-agent, vault-plugin-secrets-jwt), and any vendored Go module whose
  full vendor/ rewrite on a dependency bump would exceed what GitHub's
  git/trees API accepts in one commit

For QA (optional for docs, build, test, refactor, ci, chore, style, and revert PRs)

verified it with cloned repo: https://github.com/shivakunv/nvcf/pulls
QA not needed

Issues

closes: #2026

Checklist

• I am familiar with the Contributing Guidelines.
• I have signed off my commits for Developer Certificate of Origin (DCO) compliance.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

Summary by CodeRabbit

• Chores
  • Automated Go module dependency updates are configured to run daily at 09:00 UTC, with an option to trigger them manually.
  • Dependency update pull requests target the main branch and are grouped and labeled to make review easier.
  • Update volume is limited, and selected project paths are excluded from automated updates.
  • Minor, patch, and digest updates receive a consistent commit prefix.

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