Remove executor_ prefix from filenames in the executors package

What does this MR do?

Removes unnecessary executor_ prefix from filenames in the executors package.

Also renames executors/custom/executor.go to executors/custom/custom.go so that it follows the same convention as others - executors/<executor_name>/<executor_name>.go

Why was this MR needed?

Makes the file names more go idiomatic.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug - n/a
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Fixes #16201 (closed)