macOS: bump baseline to 10.15, fix GL_SILENCE_DEPRECATION placement

[johncurley]

resourcedata_mac.mm:

• Remove @available(macOS 10.15, *) guard around monospacedSystemFontOfSize:weight:
• Drop systemFontOfSize: fallback — 10.15 is now the minimum supported version

cocoa_display_window.mm:

• Move GL_SILENCE_DEPRECATION before OpenGL header imports so it actually silences the deprecation warnings (defining it after the headers have no effect)
• Wrap initOpenGL() NSOpenGL usage in -Wdeprecated-declarations pragmas for cleaner builds on macOS 12+

Hi dpjudas,

I've bumped the minimum version for macOS to 10.15+ from 10.13+ because the ShaderCompiler library I'm using has glsl-lang support at 10.15+ minimum. Metal renderer is feature complete, I'll post to the forums soon and as promised will start the HaikuOS porting at some stage.

[dpjudas]

I had to force push the master branch because this commit included copilot ad spam. I appreciate the PR itself, but please make sure in the future that AI companies do not inject spam into your work. :)

[johncurley]

Ah I see. No worries, I was a little lazy pushing out the last few commits for the renderer as it was failing cross platform builds so asked copilot to create a new branch. I should have checked the commit a little better.