Add GCC 12 to the build matrix.

Change-Id: Ibb389e3044c1932b908bc330a36a85dd7ad73ca8
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60110
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9bc3275..7ce3a93 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -47,7 +47,7 @@
     strategy:
       fail-fast: false
       matrix:
-        ver: [6, 7, 8, 9, 10, 11]
+        ver: [6, 7, 8, 9, 10, 11, 12]
     env:
       CC: gcc
       CXX: g++