fix: map Python CRITICAL log level to OTel FATAL severity text (#4365)

* fix: map Python CRITICAL log level to OTel FATAL severity text in contrib handler

The OTel specification defines the severity text for the highest severity
range as "FATAL", but the contrib logging handler was passing through
Python's native "CRITICAL" level name unchanged. This mirrors the fix
made in open-telemetry/opentelemetry-python#5011 for the SDK handler.

Related: open-telemetry/opentelemetry-python#4984

* Add CHANGELOG entry for CRITICAL -> FATAL severity text mapping

* Update instrumentation/opentelemetry-instrumentation-logging/src/opentelemetry/instrumentation/logging/handler.py

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Co-authored-by: Leighton Chen <lechen@microsoft.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>