[repo] cleanup #nullable disable

[Kielek]

Towards #3958

Please merge after #5900

Changes

[repo] cleanup #nullable disable + update contributing guide

There is couple (3) of leftovers of #nullable disable. It is needed for auto-generated files.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• [ ] Unit tests added/updated
• [ ] Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.30%. Comparing base (6250307) to head (552411a).
Report is 349 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5901      +/-   ##
==========================================
+ Coverage   83.38%   86.30%   +2.92%     
==========================================
  Files         297      257      -40     
  Lines       12531    11219    -1312     
==========================================
- Hits        10449     9683     -766     
+ Misses       2082     1536     -546     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.26% <ø> (?)
unittests-Project-Stable	86.21% <ø> (?)
unittests-Solution	85.95% <ø> (?)
unittests-UnstableCoreLibraries-Experimental	85.68% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...xporter.Console/Implementation/ConsoleTagWriter.cs	11.11% <ø> (ø)
....Exporter.Zipkin/Implementation/ZipkinTagWriter.cs	63.63% <ø> (ø)

... and 237 files with indirect coverage changes

[ysolomchenko]

LGTM

[CodeBlanch]

LGTM

[CodeBlanch]

@Kielek

Should we remove "nullable enabled" from here too?

opentelemetry-dotnet/.github/PULL_REQUEST_TEMPLATE.md

Line 10 in 1a68b49

* [ ] [CONTRIBUTING](https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/CONTRIBUTING.md) guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[ysolomchenko]

I think we can leave it there for new contributors who create projects and files with nullable disabled for some reason, and they can already see this line where they can read and understand that files must have nullable enabled.