feat(otlp-exporters): bump otlp proto to 0.20.0

[pichlermarc]

Which problem is this PR solving?

This PR bumps https://github.com/open-telemetry/opentelemetry-proto for the OTLP exporters to 0.20.0 (previously we were at 0.18.0).

Fixes #3870
OTLP features added by this PR will be implemented in #3392, #3183

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Unit tests
• Manual testing against latest collector (0.80.0)

[codecov]

Codecov Report

Merging #3932 (4d93c63) into main (5dc0f69) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 4d93c63 differs from pull request most recent head 035e049. Consider uploading reports for the commit 035e049 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3932      +/-   ##
==========================================
- Coverage   92.91%   92.89%   -0.03%     
==========================================
  Files         297      297              
  Lines        8838     8838              
  Branches     1815     1815              
==========================================
- Hits         8212     8210       -2     
- Misses        626      628       +2     

Impacted Files	Coverage Δ
...-grpc-exporter-base/src/LogsExportServiceClient.ts	42.85% <ø> (ø)
...pc-exporter-base/src/MetricsExportServiceClient.ts	42.85% <ø> (ø)
...grpc-exporter-base/src/TraceExportServiceClient.ts	42.85% <ø> (ø)
...mental/packages/otlp-transformer/src/logs/types.ts	100.00% <ø> (ø)
...tal/packages/otlp-transformer/src/metrics/types.ts	100.00% <ø> (ø)
...ental/packages/otlp-transformer/src/trace/types.ts	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

[pkanal]

Ty for updating this!