Add webhook rate-limit threshold for Free plan on gitlab.com

What does this MR do?

Part of #330133 (closed), see #330133 (comment 590024134) for the data behind this threshold.

The FF for this has been disabled again on gitlab.com, and we'll reenable it after the %14.0 release on June 22, and will also announce this change in the release post.

Migration output

Note: The migration only runs on gitlab.com, so this is a no-op locally:

$ rails db:migrate:up VERSION=20210601131742
== 20210601131742 UpdateWebHookCallsLimit: migrating ==========================
== 20210601131742 UpdateWebHookCallsLimit: migrated (0.0000s) =================

$ rails db:migrate:down VERSION=20210601131742
== 20210601131742 UpdateWebHookCallsLimit: reverting ==========================
== 20210601131742 UpdateWebHookCallsLimit: reverted (0.0000s) =================

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #330133 (closed)