feat: diagnostic if an incompatible language version is used

[latonz]

diagnostic if an incompatible language version is used

Description

Emits RMG046 as an error if a language version < 9.0 is used.

Fixes #532

Checklist

• The existing code style is followed
• The commit message follows our guidelines
• Performed a self-review of my code
• Hard-to-understand areas of my code are commented
• The documentation is updated (as applicable)
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)

[codecov]

Codecov Report

Merging #566 (bac1e72) into main (20fd885) will increase coverage by 0.05%.
The diff coverage is 95.23%.

❗ Current head bac1e72 differs from pull request most recent head de74127. Consider uploading reports for the commit de74127 to get more accurate results

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
+ Coverage   90.99%   91.04%   +0.05%     
==========================================
  Files         166      166              
  Lines        5606     5627      +21     
  Branches      710      711       +1     
==========================================
+ Hits         5101     5123      +22     
+ Misses        354      351       -3     
- Partials      151      153       +2     

Impacted Files	Coverage Δ
src/Riok.Mapperly/MapperGenerator.cs	88.33% <92.30%> (+1.09%)	⬆️
...Riok.Mapperly/Diagnostics/DiagnosticDescriptors.cs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[github-actions]

🎉 This PR is included in version 2.9.0-next.3 🎉

The release is available on:

• GitHub release
• v2.9.0-next.3

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 3.0.0 🎉

The release is available on:

• GitHub release
• v3.0.0

Your semantic-release bot 📦🚀