Add Google OIDC terms to capitalization list

As I learned in a related merge request yesterday, these terms must
always be capitalized. No reason not to get them added to the
enforcement list, then.

.markdownlint.yml

@@ -139,6 +139,8 @@ proper-names:
     "unicorn-worker-killer",
     "URL",
     "WebdriverIO",
+    "Workload Identity Pool",
+    "Workload Identity Provider",
     "YAML",
     "YouTrack"
   ]