schema-publish: Use commit date instead of author date

[ralfhandl]

• Use commit date of last change to construct schema file names instead of author date
  • the date a change was merged into the spec version dev branch is easier to track/understand than the date the author made that change in their local repo
• Improve console output of what happened why

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• no schema changes are needed for this pull request

[karenetheridge]

This should be today's date (the date that the schemas are published), not the date of the most recent merge.

[ralfhandl]

This should be today's date (the date that the schemas are published), not the date of the most recent merge.

There are four schema files, and so far we only create a new iteration of a file if

• its source changes, or
• one of its dependencies changes.

Then we use the date of that change as the iteration name.

This minimises the number of iterations:

• If schema.yaml changes, only schema and schema-base get new iterations with the same date.
• If meta.yaml changes, all four files get new iterations with the same date.

This PR keeps that logic and only picks a "better" change date - the commit date.