Add GCC 11 to the build matrix.

Change-Id: I04637de40a3f34dadb1c512c35aaa126398d8a3e
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58530
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9ac25ff..09d1687 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -41,7 +41,7 @@
     strategy:
       fail-fast: false
       matrix:
-        tag: [4, 5, 6, 7, 8, 9, 10]
+        tag: [4, 5, 6, 7, 8, 9, 10, 11]
     env:
       CC: gcc
       CXX: g++