Make maintained PoA bootnode connections configurable

[matthew1001]

PR description

This PR follows on from #7257. Specifically it makes the behaviour configurable and defaults to off. Since the 25.7.0 release has just gone out this PR will technically be a change in behaviour, so I'll add a change log entry accordingly. However, assuming this PR is included in an upcoming release I don't imagine any users will be affected by the behaviour change.

In an enterprise network with a large number of nodes, maintaining connections to the bootnodes could be problematic for the bootnodes. Defaulting the behaviour to off avoids the need to specifically disable the behaviour on all of the nodes, but allows a smaller chain to be configured to still treat bootnodes like static nodes.

Locally, you can run these tests to catch failures early:

• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests

[matthew1001]

After discussion with @pinges we've decided to close this and revert #7257 (the latter has gone into 24.7.0 so will need a breaking change entry). #7314 provides the behaviour change that is most important - namely that bootnodes can be retried during peer table refresh - so that PR will merge pending approval.