Update gitignore to avoid codeql metadata files

Also, we're no longer using the rush package manager, so delete that
from gitignore.

Author: aeisenberg

.gitignore

@@ -16,7 +16,6 @@ artifacts/
 # Visual Studio workspace state
 .vs/
 
-# Rush files
-/common/temp/**
-package-deps.json
-**/.rush/temp
+# CodeQL metadata
+.cache/
+.codeql/