[sdk-logs] ExperimentalAttribute decorations for experimental log APIs

[CodeBlanch]

Follow-up to #5148

Changes

• Adds .NET 8 ExperimentalAttribute decoration for experimental log APIs in SDK.

Merge requirement checklist

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

[codecov]

Codecov Report

Merging #5151 (2205ac1) into main (2a850d6) will increase coverage by 0.04%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5151      +/-   ##
==========================================
+ Coverage   83.03%   83.07%   +0.04%     
==========================================
  Files         297      297              
  Lines       12380    12380              
==========================================
+ Hits        10280    10285       +5     
+ Misses       2100     2095       -5     

Flag	Coverage Δ
unittests	83.07% <ø> (+0.04%)	⬆️

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

Files	Coverage Δ
...c/OpenTelemetry.Api/Logs/LogRecordAttributeList.cs	68.14% <ø> (ø)
src/OpenTelemetry.Api/Logs/LogRecordData.cs	100.00% <ø> (ø)
...nTelemetry.Api/Logs/LogRecordSeverityExtensions.cs	100.00% <ø> (ø)
src/OpenTelemetry.Api/Logs/Logger.cs	100.00% <ø> (ø)
src/OpenTelemetry.Api/Logs/LoggerProvider.cs	100.00% <ø> (ø)
...ry/Logs/Builder/LoggerProviderBuilderExtensions.cs	100.00% <ø> (ø)
...try/Logs/ILogger/OpenTelemetryLoggingExtensions.cs	95.23% <ø> (ø)
src/OpenTelemetry/Logs/LogRecord.cs	67.37% <ø> (ø)
src/OpenTelemetry/Logs/LoggerProviderExtensions.cs	86.66% <ø> (ø)
src/OpenTelemetry/Sdk.cs	100.00% <ø> (ø)

... and 5 files with indirect coverage changes