[19.1] Use the system CA bundle for gRPC default roots

What does this MR do and why?

Backport of Use the system CA bundle for gRPC default roots.

Points gRPC's default CA roots at Gitlab::X509::Certificate.ca_certs_bundle — the same CA bundle every other gRPC client in the codebase uses — instead of gRPC's own vendored etc/roots.pem, in config/initializers/grpc.rb. The bundled gRPC roots update on gRPC's own release schedule and can lag the system trust store; the gem we vendor predates the eMudhra/emSign CA distrust, so gRPC clients using default credentials (e.g. the Cells Topology Service client and the Google Artifact Registry client) would verify against a stale root set. With this change, both Omnibus and CNG gRPC TLS honor the system/embedded cacert.pem and any admin-added trusted-certs.

This is a clean cherry-pick of the single source commit; both files are new, so there are no conflicts on any stable branch.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.