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

Build Catch3 as C++17

Version 1.1 left programs built with Progmasoft tools under MPL-2.0 duties
for the code the tools place in them, and left other cases unclear.
Version 2.0 adds a generated code permission, covers runtime and standard
libraries, compiler transformations, interfaces and examples, and settles
how a file is identified and how to tell recipients about source.

Every project SPDX expression moves to AdditionRef-Progmasoft-Exception-2.0.
The older texts stay, because released versions name them.

A lambda in the POSIX half of Process.cpp captured a structured binding,
which needs C++20 and which a Windows build never compiles. The files in
extras still held the C++20 sources, so they are generated again, and the
generator no longer says the runner needs C++20.