Remove cert-manager release-1.17 jobs and refs

cert-manager 1.17 is no longer supported so we can remove all the testing
infrastructure.

- Remove release-1.17 prow job spec and associated periodics
- Remove release-1.17 branch-protection and testgrid entries
- Drop release-1.17 from prowgen knownBranches
- Bump cert-manager helm install version to v1.18.2
- Adjust cron schedules for release-1.18 and release-1.19 periodics; a
- side-effect of running `make prowgen`.

Part of:
 * cert-manager/cert-manager#8113

Signed-off-by: Richard Wall <[email protected]>

Author: wallrj-cyberark

config/config.yaml

@@ -80,13 +80,6 @@ branch-protection:
             # branch basis, since context names differ. Post release, the
             # release branches can be updated to allow for cherry picks to the
             # latest release.
-            release-1.17:
-              required_status_checks:
-                contexts:
-                - pull-cert-manager-release-1.17-make-verify
-                - pull-cert-manager-release-1.17-make-test
-                - pull-cert-manager-release-1.17-e2e-v1-32
-                - pull-cert-manager-release-1.17-e2e-v1-32-upgrade
             release-1.18:
               required_status_checks:
                 contexts: