* Bazel: general improvements.
Improve the Bazel build. Attempt to fix previous errors recorded while
trying to publish ftxui in the Bazel Central Registry:
- https://github.com/bazelbuild/bazel-central-registry/pull/4485
- https://buildkite.com/bazel/bcr-presubmit/builds/13601#01968b61-f5b2-4d16-94d0-c87a03a1a23b
Test against "recent" platforms
-------------------------------
Previously, I got the error:
```
gcc: error: unrecognized command line option '-std-c++20'; did you mean '-std-c++2a'?
```
This was due to using old distribution like ubuntu 2004. Test against
newer platforms only to avoid GCC version<-9.x.y
Downgrade gtest version.
------------------------
I suspect this caused the Bazel Central Registry error:
```
file:///workdir/modules/googletest/1.15.2/MODULE.bazel:68:20: name 'use_repo_rule' is not defined
```
Specifying using bazelmod fixes the issue. Thanks @robinlinden
Tag gtest as dev_dependency
---------------------------
Presumably, this should avoid dependants to fetch it?
Enable --features-layering_check
--------------------------------
Aka clang `-Wprivate-header`. Fix the encountered errors.
Use clang in the CI
-------------------
The CI was defining clang/gcc in the matrix, but was not using it. Fix
the bug.
