Update plugin template to support OBS Studio 31.1.1

[PatTheMav]

Description

This PR updates open issues of the plugin template that have arisen since the last major release of OBS Studio:

• Updates build specification to use OBS Studio 31.1.1 and associated dependencies (obs-deps and Qt6)
• Updates OBS Studio build flags according to upstream renaming of "UI" to "frontend" to ensure that no frontend targets are built to generate the libobs library
• Updates clang-format to use version 19.1.1 to stay in sync with OBS Studio
• Updates the clang-format rules to stay in sync with OBS Studio
• Removes the macOS host check script as the check is implemented in CMake already, also removes the maintenance burden of keeping both values in sync manually.
• macOS deployment target is increased to 12.0 in sync with Qt 6.8 and OBS Studio
• Adds the missing assignment of the code sign team input to the CI build action

Motivation and Context

While there are open pull requests to fix some of those issues, they are either incomplete or require further work. In the interest of getting these issues fixed quickly I chose to fix them directly.

Fixes #146
Supersedes #149
Supersedes #150

How Has This Been Tested?

Successfully built a test plugin based on the updated template on macOS 15.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

Looks fine at a glance.