Rename setEpoch to setTimestamp

[jack-berg]

Was adding support observed timestamp and I noticed LogRecordBuilder#setEpoch is poorly named. The log record data model has two timestamp fields (timestamp and observed timstamp), and setEpoch is ambiguous about which is being set.

Let's get this resolved before stabilizing. I propose we rename to setTimestamp, which is more clear.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (ecc41c7) 91.18% compared to head (0b29507) 91.20%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5368      +/-   ##
============================================
+ Coverage     91.18%   91.20%   +0.01%     
  Complexity     4865     4865              
============================================
  Files           546      546              
  Lines         14369    14373       +4     
  Branches       1351     1351              
============================================
+ Hits          13103    13109       +6     
  Misses          875      875              
+ Partials        391      389       -2     

Impacted Files	Coverage Δ
.../java/io/opentelemetry/api/logs/DefaultLogger.java	100.00% <ø> (ø)
...va/io/opentelemetry/api/logs/LogRecordBuilder.java	60.00% <ø> (ø)
...y/exporter/logging/SystemOutLogRecordExporter.java	92.85% <100.00%> (ø)
...etry/exporter/internal/otlp/logs/LogMarshaler.java	67.10% <100.00%> (ø)
...metry/sdk/testing/assertj/LogRecordDataAssert.java	100.00% <100.00%> (ø)
...ntelemetry/sdk/testing/logs/TestLogRecordData.java	100.00% <100.00%> (ø)
...io/opentelemetry/sdk/logs/SdkLogRecordBuilder.java	97.61% <100.00%> (+0.18%)	⬆️

... and 2 files with indirect coverage changes

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[breedx-splk]

❤️ Love it. Thank you! Easy win.