[AOT] Added net7.0 to the TargetFrameworks to all projects (except for OTLP Exporter) under src folder.

[Yun-Ting]

Changes

Towards #3429
Added net7.0 to the TargetFrameworks to all projects (except for OTLP Exporter) under src folder because more APIs were annotated with RequiresDynamicCode in net7.0: #3429 (comment)
Will use a follow-up PR to fix errors when building net7.0 for OTLP Exporter and add net7.0 as the target for it.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

Merging #4370 (316ae45) into main (7db6941) will increase coverage by 0.02%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4370      +/-   ##
==========================================
+ Coverage   84.75%   84.77%   +0.02%     
==========================================
  Files         300      300              
  Lines       12010    12010              
==========================================
+ Hits        10179    10182       +3     
+ Misses       1831     1828       -3     

Impacted Files	Coverage Δ
src/OpenTelemetry.Api/Baggage.cs	100.00% <ø> (ø)
src/OpenTelemetry.Api/Trace/Tracer.cs	97.22% <ø> (ø)

... and 4 files with indirect coverage changes