Break backward compatibility for configuration file location in API Fuzzing

Problem

API Fuzzing has maintained backwards compatibility for the default configuration file. The current default is .gitlab/.gitlab-api-fuzzing.yml chagned from .gitlab-api-fuzzing.yml.

Backwards compatibility will end in 14.0.

References #273812 (closed)

Proposal

Remove backwards compatibility for old configuration file location.

1. [-] Remove backwards compatibility from worker-entry
2. Remove FUZZAPI_CONFIG variable from template
3. Remove FUZZAPI_CONFIG from documentation (!61047 (merged))
4. [-] Remove tests

/cc @sethgitlab @stkerr