feat(mandatory-jobs): increase runner resources for failing SAST jobs

The gitlab-advanced-sast and kics-iac-sast jobs are currently failing due to not having enough resources since being switched to use the GKE devops-general runner. This commit explicitly sets increased CPU and memory requests/limits to resolve the issue.

Example pipeline showing two gitlab-advanced-sast jobs, the failing one does not have the new limits set and the passing one does - https://gitlab.developers.cam.ac.uk/uis/devops/iam/activate-account/infrastructure/-/pipelines/866101

Note

The first commit in the MR is a simple refactor to make the yaml a bit more DRY by using an anchor to reuse the runner tag string, rather than duplicating the string value.

Part of https://gitlab.developers.cam.ac.uk/uis/devops/devhub/gitlab-runner-infrastructure/-/issues/88