fix!: removal of already required system fields

[shumkov]

Issue being fixed or feature implemented

Currently, we do not support making optional already required document system fields such as $createdAt, $createdAtHeight, etc. This will break the state and ST processing logic.

What was done?

• Do not allow to remove fields started from $ on from required list on data contract update.

How Has This Been Tested?

Added a test that expects an error on removal.

Breaking Changes

Data contract update validation rules are changed. Previous blockchain data might not be valid.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone