SemanticConventions: update link to v1.21.0 tag

[TimothyMothra]

Design discussion issue #4484

Changes

• Update all semantic convention links to the v1.21.0 tag.
  The semantic-conventions repo completed their first release of v1.21.0.
  I would rather link to a tagged version of the specs than the main branch.

Please provide a brief description of the changes here.

Merge requirement checklist

• CONTRIBUTING guidelines followed (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

Merging #4659 (83ae531) into main (18f7978) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 83ae531 differs from pull request most recent head 1be5a56. Consider uploading reports for the commit 1be5a56 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4659      +/-   ##
==========================================
+ Coverage   84.96%   85.00%   +0.03%     
==========================================
  Files         314      314              
  Lines       12715    12715              
==========================================
+ Hits        10803    10808       +5     
+ Misses       1912     1907       -5     

Impacted Files	Coverage Δ
...tation.AspNetCore/Implementation/HttpInListener.cs	90.47% <ø> (ø)
...AspNetCore/Implementation/HttpInMetricsListener.cs	64.70% <ø> (ø)
...tp/Implementation/HttpHandlerDiagnosticListener.cs	68.42% <ø> (ø)
...ementation/HttpHandlerMetricsDiagnosticListener.cs	75.75% <ø> (ø)
...plementation/HttpWebRequestActivitySource.netfx.cs	80.86% <ø> (ø)
...ation.SqlClient/SqlClientInstrumentationOptions.cs	98.87% <ø> (ø)
src/Shared/HttpSemanticConventionHelper.cs	80.00% <ø> (ø)

... and 5 files with indirect coverage changes