Most of these will not matter in practice due to C++14 imposing
significant limitations on what else we can make constexpr, and we cannot
have references outliving the constexpr context either way.

GCC 13 (as usual for new compiler releases) shuffles around some
internal includes and so <cstdint> is no longer transitively included.

Explicitly include <cstdint> for uint64_t.

```
FAILED: src/CMakeFiles/Catch2.dir/catch2/internal/catch_clara.cpp.o
/usr/lib/ccache/bin/g++-13  -I/var/tmp/portage/dev-cpp/catch-3.0.1/work/Catch2-3.0.1/src/catch2/.. -I/var/tmp/portage/dev-cpp/catch-3.0.1/work/Catch2-3.0.1_build/generated-includes  -O2 -pipe
-march=native -fdiagnostics-color=always -frecord-gcc-switches -Wreturn-type -D_GLIBCXX_ASSERTIONS  -ggdb3 -fdiagnostics-color=always
-ffile-prefix-map=/var/tmp/portage/dev-cpp/catch-3.0.1/work/Catch2-3.0.1=. -Wall -Wc++20-compat -Wcast-align -Wcatch-value -Wdeprecated -Wexceptions -Wextra -Wextra-semi -Wfloat-equal -Winit-self
-Wmisleading-indentation -Wmismatched-new-delete -Wmismatched-tags -Wmissing-braces -Wmissing-declarations -Wmissing-noreturn -Wnull-dereference -Wold-style-cast -Woverloaded-virtual -Wparentheses
-Wpedantic -Wreorder -Wshadow -Wstrict-aliasing -Wsuggest-override -Wundef -Wuninitialized -Wunreachable-code -Wunused -Wunused-function -Wunused-parameter -Wvla -MD -MT
src/CMakeFiles/Catch2.dir/catch2/internal/catch_clara.cpp.o -MF src/CMakeFiles/Catch2.dir/catch2/internal/catch_clara.cpp.o.d -o src/CMakeFiles/Catch2.dir/catch2/internal/catch_clara.cpp.o -c
/var/tmp/portage/dev-cpp/catch-3.0.1/work/Catch2-3.0.1/src/catch2/internal/catch_clara.cpp
In file included from /var/tmp/portage/dev-cpp/catch-3.0.1/work/Catch2-3.0.1/src/catch2/internal/catch_clara.cpp:12:
/var/tmp/portage/dev-cpp/catch-3.0.1/work/Catch2-3.0.1/src/catch2/../catch2/internal/catch_string_manip.hpp:47:14: error: 'uint64_t' in namespace 'std' does not name a type; did you mean 'wint_t'?
   47 |         std::uint64_t m_count;
      |              ^~~~~~~~
      |              wint_t
/var/tmp/portage/dev-cpp/catch-3.0.1/work/Catch2-3.0.1/src/catch2/../catch2/internal/catch_string_manip.hpp:51:42: error: expected ')' before 'count'
   51 |         constexpr pluralise(std::uint64_t count, StringRef label):
      |                            ~             ^~~~~~
      |                                          )
```

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

There is no good reason to provide a "add empty line" primitive
for writing XML documents, and the fact that it remains unused
after all the time it was provided only confirms this further.

Previously, string literals and `std::string`s would match the
template variant, which would serialize them into a stream and then
call the `StringRef` overload for resulting string. This caused
bunch of codebloat and unnecessary pessimization for common usage.