ci: Test ubuntu 26.04 container

Author: taiki-e

.github/workflows/ci.yml

@@ -135,6 +135,7 @@ jobs:
           - ubuntu:20.04 # glibc 2.31
           - ubuntu:22.04 # glibc 2.35
           - ubuntu:24.04 # glibc 2.39
+          - ubuntu:26.04 # glibc 2.42
           # - debian:9-slim # glibc 2.24 # https://github.com/actions/runner/issues/2906
           - debian:10-slim # glibc 2.28
           - debian:11-slim # glibc 2.31