testing: update CI configurations (#1846)

* testing: remove unnecessary prerelease targets

This PR does two things:
* remove unneeded prerelease-deps configs for removed and nonexisting CI targets
* fixes the continuous prerelease-deps-3.12 config

Author: shollyman

.kokoro/continuous/prerelease-deps-3.11.cfg renamed to .kokoro/continuous/prerelease-deps-3.12.cfg

@@ -3,5 +3,5 @@
 # Only run this nox session.
 env_vars: {
     key: "NOX_SESSION"
-    value: "prerelease_deps-3.11"
+    value: "prerelease_deps-3.12"
 }