[release/v1.21.x] Prepare release 1.21.0

[opentelemetrybot]

Prepare release 1.21.0.

[codecov]

Codecov Report

Base: 91.27% // Head: 91.20% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (03c820d) compared to base (ca18354).
Patch has no changes to coverable lines.

Additional details and impacted files

@@                  Coverage Diff                  @@
##             release/v1.21.x    #5035      +/-   ##
=====================================================
- Coverage              91.27%   91.20%   -0.07%     
+ Complexity              4882     4880       -2     
=====================================================
  Files                    552      552              
  Lines                  14424    14424              
  Branches                1375     1375              
=====================================================
- Hits                   13165    13156       -9     
- Misses                   874      883       +9     
  Partials                 385      385              

Impacted Files	Coverage Δ
.../io/opentelemetry/api/internal/PercentEscaper.java	79.69% <0.00%> (-4.52%)	⬇️
...metry/sdk/metrics/export/PeriodicMetricReader.java	87.14% <0.00%> (-2.86%)	⬇️
...telemetry/sdk/trace/export/BatchSpanProcessor.java	91.72% <0.00%> (-0.69%)	⬇️

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