ci: switch away from EOL'd Ubuntu version in linux-exotic

The "linux-exotic" job still uses Ubuntu 20.04, which is end of life
nowadays and starting to show some cracks. Switch it over to use the
"latest" tag instead so that we don't have to constantly update it
anymore.

Now arguably, this reduces test coverage for old versions of Ubuntu.
But "latest" at least points to the most up-to-date LTS release of
Ubuntu, compared to the "rolling" tag that uses the latest release
regardless of the LTS status. So while "latest" and "rolling" are the
same right now, that's not always the case.

Furthermore, we have other jobs that test with ancient versions of
Linux, like for example the one that uses AlmaLinux 8 (2021, originally
tracking RHEL 8 from 2019) or Debian 12 (2023).

Note that this also requires us to switch away from GCC 8, which is not
supported by Ubuntu 26.04 anymore. Instead we simply use the default
version of GCC.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -409,9 +409,7 @@ jobs:
           image: ubuntu:rolling
           cc: clang
         - jobname: linux-exotic
-          image: ubuntu:20.04
-          cc: gcc
-          cc_package: gcc-8
+          image: ubuntu:latest
         - jobname: linux-breaking-changes
           cc: gcc
           image: ubuntu:rolling

.gitlab-ci.yml

@@ -43,9 +43,7 @@ test:linux:
         image: ubuntu:rolling
         CC: clang
       - jobname: linux-exotic
-        image: ubuntu:20.04
-        CC: gcc
-        CC_PACKAGE: gcc-8
+        image: ubuntu:latest
       - jobname: linux-breaking-changes
         image: ubuntu:rolling
         CC: gcc