docs: dd-trace 5.0 to 6.0 - document startup logs default

[mohab-sameh]

Summary

Document that DD_TRACE_STARTUP_LOGS defaults to true in dd-trace v6 and
call out the manual decision to set DD_TRACE_STARTUP_LOGS=false when teams
need to preserve prior silence.

Detection Criteria

This is not a codemod-eligible source transformation by default. A review can
look for projects that do not explicitly configure DD_TRACE_STARTUP_LOGS.

Transformation Logic

Do not automatically inject DD_TRACE_STARTUP_LOGS=false. Add documentation or
a migration checklist item so service owners decide whether startup logs should
remain enabled.

Before / After Example

Before: no DD_TRACE_STARTUP_LOGS setting.

Manual action: set DD_TRACE_STARTUP_LOGS=false only if the service must preserve
the old quieter startup behavior.

Notes / Edge Cases

This is an operational policy choice, not a mechanical source rewrite. The
correct answer can vary by service and environment.