config: drop dual-write, replace 'authorization' with 'BearerToken'

Address review feedback: the openapi-generator v6.6.0 upgrade
declared this rename as a breaking change in the project CHANGELOG,
so the loaders should follow the rename rather than preserve the
v35 key for backward compatibility.

Removes the api_key['authorization'] write in all three affected
loaders (sync incluster, sync kube_config, async incluster) and
updates the corresponding test assertions to read 'BearerToken'.