Draft: Hard code the version requirement; messages is optional

What does this MR do?

This MR is the basis of a new schema release: version 3.0.0. This version is based off version 3.0.0-rc1, with the following changes:

• The version.pattern has been replaced with an enum containing only 3.0.0.
• scan.messages has been declared an optional field.

This new schema release will make it easier for common-based analyzers to conform to a version of the schema.

Why is this MR a draft?

This MR should never be merged to master. Instead, following a successful review process, it will be tagged, a release will be created off the tag, and the branch will be deleted closing this MR.

Why has the verify-versioning job been removed?

The 3.0.0-rc1 release is actually released with version 3.0.0 in the schema JSON file. Ideally, it would have been released with -rc1 on the end, meaning that there would be no conflict.

I don't recommend retrospectively changing the -rc1 releases, hence this conflict is considered acceptable.

Why hasn't the CHANGELOG been updated?

There is already a 3.0.0 described in the CHANGELOG. As this is not going to be merged into master, this CHANGELOG would be removed even if changes were added.