Move rate limits to application_settings.rate_limits

We need to add configuration for various application rate limits in lib/gitlab/application_rate_limiter.rb

Today we do this by adding a new column in ApplicationSetting.

Problem

We have too many columns in ApplicationSetting: 539 columns in total today

Proposal

Move such *_limit columns to the application_settings.rate_limits column introduced in Make delete members API rate limit configurable (!140633 - merged) where we store all the rate limits in a single JSONB column:

$ git grep -oh application_settings.*_limit lib/gitlab/application_rate_limiter.rb | sort | uniq
application_settings.gitlab_shell_operation_limit
application_settings.group_download_export_limit
application_settings.group_export_limit
application_settings.group_import_limit
application_settings.issues_create_limit
application_settings.notes_create_limit
application_settings.pipeline_limit
application_settings.project_download_export_limit
application_settings.project_export_limit
application_settings.project_import_limit
application_settings.projects_api_rate_limit
application_settings.raw_blob_request_limit
application_settings.search_rate_limit
application_settings.users_get_by_id_limit