Bumps .NET version to .NET9; Adds PS7.5 workflow; Bumps InvokeBuild to v5.12.0; Bumps MkDocs to v1.6.1

[mdaneri]

Description:
This update improves the build process by dynamically selecting the appropriate .NET versions to deliver binaries, . By automatically excluding unsupported .NET versions, the build.ps1 script now ensures that only maintained versions are targeted for binary generation.

Key Changes:

• Removing redundant part of code

[Badgerati]

I just feel like this is overcomplicating the build process, when all we need to support is simply netstandard2.0, net8.0, and net9.0 (removing net6.0/PS7.2 now they're EOL).

For something that will only be affected once a year, I just feel it's not required. That, and it still requires manual intervention anyway, in the csproj file and workflows.

I'm happy for this to be the net9.0/PS7.5 bump, and net6.0/PS7.2 removal.

[mdaneri]

Should I roll back to the previous code?
the csproj doesn't require any manual change for the next 3-4 years

[Badgerati]

I still feel this is way overengineering something that only takes ~10-20mins once a year to update; plus manually we can properly verify the version bump works, rather than get any nasty surprises.

[mdaneri]

Ok, I'll drop the eol part
I'm going to use a predetermined array of string

[Badgerati]

LGTM

[Badgerati]

Note: removed "Bumps MkDocs Material Theme to v9.5.44" from title for release notes, as #1490 bumps the version higher to v9.6.4.