Update MTLS tests to use high capacity runners

.gitlab/ci/jobs/mtls.gitlab-ci.yml

@@ -2,6 +2,7 @@ ce:mtls:
   extends:
     - .rules:ce-never-when-triggered-by-feature-flag-definition-change
     - .test
+    - .high-capacity
     - .ce-variables
     - .rspec-report-opts
   variables:
@@ -11,6 +12,7 @@ ee:mtls:
   extends:
     - .rules:ee-never-when-triggered-by-feature-flag-definition-change
     - .test
+    - .high-capacity
     - .ee-variables
     - .rspec-report-opts
   variables: