fix: report special opt rule as warning instead of error

[ilbertt]

Description

Following up on https://github.com/dfinity/candid/pull/648, we now show a warning when the new Candid declaration is a subtype of the old Candid declaration via a special opt rule (see https://github.com/dfinity/candid/blob/c7659ca/spec/Candid.md#upgrading-and-subtyping for more details).

Note: we should not merge this until a new Candid version with the mentioned fix is available.

How Has This Been Tested?

Same tests should pass.

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [x] I have edited the CHANGELOG accordingly.
• [ ] I have made corresponding changes to the documentation.