Add Clang 13 to the build matrix.

Change-Id: I1ec47a4f9777ff4f34aaeb0c80e7b9d61243640d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59190
Reviewed-by: Paul Wankadia <junyer@google.com>

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        tag: [9, 10, 11, 12]
+        tag: [9, 10, 11, 12, 13]
     env:
       CC: clang-${{ matrix.tag }}
       CXX: clang++-${{ matrix.tag }}