Bazel: general improvements. (#1043)

* 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.

MODULE.bazel

@@ -1,9 +1,13 @@
-# FTXUI Module.
-module(name = "ftxui", version = "6.1.8", compatibility_level = 6)
+# FTXUI module.
+module(
+    name = "ftxui",
+    version = "6.1.8",
+    compatibility_level = 6,
+)
 
-# Build deps.
+# Build dependencies.
 bazel_dep(name = "rules_cc", version = "0.1.1")
-bazel_dep(name = "platforms", version = "0.0.11")
+bazel_dep(name = "platforms", version = "0.0.10")
 
-# Test deps.
-bazel_dep(name = "googletest", version = "1.16.0.bcr.1")
+# Test dependencies.
+bazel_dep(name = "googletest", version = "1.14.0.bcr.1", dev_dependency = True)