Remove PCRE2_STATIC_RUNTIME

[NWilson]

Since we have raised our minimum CMake version to 3.15, this updates the default value of the "CMP0091" backwards- compatibility policy, such that PCRE2_STATIC_RUNTIME is now broken (since 10.45 when cmake_minimum_required was increased).

We can remove this flag, since the new built-in CMAKE_MSVC_RUNTIME_LIBRARY provides the same behaviour.

Fixes #752

[carenas]

Need to also update the bat CI job in dev.yml so it will continue testing this configuration.

Bonus points for making sure that the test actually verifies the static runtime was actually linked with by the produced artifacts so it is not silently broken, and maybe tell cmake in the Configure step to fail when an "unknown variable" warning is triggered to make its failure more visible.

[NWilson]

This CMake flag is not currently covered by any tests, so if I did add a test, it would be a new one.

Given that the new flag is a built in CMake feature, I'm not sure we need to test it ourselves continuously. I will do a final manual test before merging. Is that ok?

+My mistake! Yes, we are testing it currently. My apologies.+

I can also ensure that the CMake calls in CI are treating warnings as errors.

[NWilson]

I have updated the dev.yml file. I have done a manual test that CMake's new built-in option works, using dumpbin.exe.

I have opened another PR with improvements to our CMake warnings.