opentelemetry-instrumentation-logging: fix documented name of code attributes env var (#4912)

* opentelemetry-instrumentation-logging: fix documented name of code attributes env var

The envvar directive in constants.py documented this option as
OTEL_PYTHON_CODE_ATTRIBUTES, but the instrumentation reads
OTEL_PYTHON_LOG_CODE_ATTRIBUTES, so following the published docs had no
effect. The surrounding prose in the same module, environment_variables.py,
and the test suite all use the LOG_ form.

Written with assistance from Claude Opus 5 (Anthropic), which located the
mismatch, verified it against the installed package, and drafted this change.

* Add changelog entry for #4912

---------

Co-authored-by: krrishapatel <krrishapatel@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>