[exporter/awsxray] Change exporter.awsxray.skiptimestampvalidation feature gate from Alpha to Beta

[jj22ee]

Description:

Change exporter.awsxray.skiptimestampvalidation feature gate from Alpha to Beta. This will change the awsxrayexporter to not drop invalid trace ids (first 32-bits of trace id in Unix epoch time is not within past 30 days) by default.

Link to tracking Issue:
N/A

Testing:
Updated Unit tests.

Local testing:
Used Golang OTel instrumented sample app
Built the ADOT Collector locally with the timestamp check removal change, and ran locally with feature gate enabled

Scenarios tested with XRay Service (drops W3C trace):

• Removed the ID Generator in Golang sample app, created a few traces with W3C trace ID
  • result: Collector logs indicate that XRay has dropped the segments
• Created a batch of traces with W3C trace ID and XRay trace ID
  • result: Collector logs indicate that XRay has dropped only the w3c segments

Example logs from XRay logged by awsxrayexporter due to invalid trace IDs sent to XRay:

2023-09-07T17:29:30.495Z	debug	[email protected]/awsxray.go:79	response: {
  UnprocessedTraceSegments: [{
      ErrorCode: "InvalidTraceId",
      Id: "33f5358b5290fbc8",
      Message: "Invalid segment. ErrorCode: InvalidTraceId"
    },{
      ErrorCode: "InvalidTraceId",
      Id: "6be91641d130c356",
      Message: "Invalid segment. ErrorCode: InvalidTraceId"
    }]
}	{"kind": "exporter", "data_type": "traces", "name": "awsxray"}

Documentation:
Updated awsxrayexporter README to indicate that the exporter will not drop the traces with invalid trace ids anymore. Let X-Ray handle the logic for dropping the traces instead.

[jj22ee]

@bogdandrutu @djaglowski pinging for approval and merge