GSTG - Bump zoekt indexer to v0.5.1

What does this MR do?

This MR bumps zoekt indexer image tag to v0.5.1 in GSTG, which was merged in Bump zoekt indexer to v0.5.1 (gitlab-org/build/CNG!1772 - merged)

This also adds ZOEKT_INDEXER_CONCURRENCY_LIMIT to limit the concurrency to prevent OOM from happening.

Author Check-list

Please read the Contributing document and once you do, complete the following:

• Check if all of the following apply:
  • Assign to the correct reviewer per the contributing document
  • Apply the correct metadata per the contributing document
  • Link to related MRs for applying the changes on other environments
  • Link to related Chef changes
  • If necessary link to a Criticality 4 Change Request issue

Reviewer Check-list

• Check if all of the following apply:
  • Reviewed the diff jobs to confirm changes are as expected
  • No changes shown in the diffs not associated with this MR - This may require a rebase or further investigation

Applier Check-list

• Make sure there is no ongoing deployment for the affected envs before merging (see #announcements slack channel)