Release Catch3 0.3.0

The release notes get their date and name the tag prefix Catch3 uses from
now on. Every other version location already says 0.3.0; MODULE.bazel was
the one left.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Say C++17 everywhere the project describes itself

The move to C++17 left a trail of older claims. The Runner's own
.clang-format still said c++20; the result is byte for byte the same under
either value. Premake and Xmake still compiled the library as C++20 and
the main library as C++14, which meant their CI jobs were not testing the
language the project promises. Both now use C++17 for every target.

The rest is text: the Meson pkg-config description claimed C++14, comments
in the CMake and Meson files explained requirements that no longer exist,
.clang-tidy gave C++14 as the reason for two exclusions, issue templates
asked for C++20 samples, and the example headers named -std=c++14 and
catch2-with-main. The release notes and the migration guide state the
C++17 minimum. Doxygen's brief called the project "Catch2's test API",
which presents it as Catch2. Meson declared only BSL-1.0; it now names both
licenses of the tree. The amalgamated files are regenerated.

Keep the unused operator quiet under clang-cl too

The test silenced -Wunused-function behind __GNUC__, which clang-cl does not
define, so that build printed the warning.

Convert to double in the coverage report explicitly

Dividing a long long by 10.0 is an implicit conversion, which fails in a
project that builds its tests with -Wimplicit-int-float-conversion and
warnings as errors, as Tala does.

Move to the Progmasoft Linking Exception, Version 2.0