Core: Replace C math headers with C++ equivalents

[Repiteo]

• Related: Remove ABS in favor of Math::abs #69406
• Related: Core: Use Math functions where possible #96498

One of the discussions in the Math::abs PR noted that we were still using the C API for quite a lot of our math operations, rather than C++ bindings. This PR makes the first major step to remedy this, by expunging math.h from the core entirely—in favor of cmath. It may persist via third-party includes, but ideally this will allow us to transition to the more modern syntax & better evaluate the areas of code that were using those C-headers instead of our math header.

Note that, in constrast to #96498, this simply elevates the global functions to the std namespace, NOT the Math class. Trying to integrate everything into Math would require more invasive changes, and ultimately the priority was deprecating the C-Header, so that can be addressed in a followup PR.

[lawnjelly]

Most of these should be using the Godot Math:: functions I think, rather than std::.

What's the benefit of converting them to std::, and then to Math:: in another PR, versus just changing them to Math::?

[Repiteo]

If we'd rather go straight to Math::, we could just use #96498 instead. The only caveat there is it's built upon the Math namespace PR, which should be approved/merged first.