Ensure new Releases always have `author_id`

What does this MR do and why?

Add validation on author_id for new release records.

Also fixes the Github importer and legacy Github importer which were importing releases without author_id.

Records created before %11.7 will continue to have empty author_id. We are performing backfil DB migration because of reasons listed out in #343448 (comment 965640386)

• legacy reason 1ea2d9fa
• Foreign key relation with User table i.e fk_8e4456f90f

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How to set up and validate locally

Rails console

> Feature.enable(:validate_release_with_author)
> release = Release.new(project: Project.last, author_id: nil, name: 'Release 1.0', tag: 'V1.0')
> release.save!

ActiveRecord::RecordInvalid: Validation failed: Author can't be blank
from /Users/bala/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/activerecord-6.1.4.7/lib/active_record/validations.rb:80:in `raise_validation_error'

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #343448 (closed)