allow snap/checkpoint with clique

[macfarla]

Docs - can't see anywhere in the docs where we mention snap or checkpoint on the private networks side, but may be worth adding an explicit note that you must use FAST or FULL if using BFT consensus.

Thanks for sending a pull request! Have you done the following?

• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Most advanced CI tests are deferred until PR approval, but you could:

• locally run all unit tests via: ./gradlew build
• locally run all acceptance tests via: ./gradlew acceptanceTest
• locally run all integration tests via: ./gradlew integrationTest
• locally run all reference tests via: ./gradlew ethereum:referenceTests:referenceTests

PR description

Fixed Issue(s)

Adjustment to #6625
Fixes #6385