Mark priority mempool as deprecated in v0.37.x

[hvanz]

We need to mark the priority mempool as deprecated, as decided in #39.

Since these changes only affect the 0.37 release, we should make the PR on v0.37.x, instead of main.

Tasks

• Add comments with special Deprecated label to relevant public functions.
• Log a warning message on node startup when "v1" is the mempool version set in the config. For that, update CheckDeprecate() in config/config.go.
• Update docs

Definition of Done

• Implementation code complete and all PRs merged
• Any changes needed to the documentation/spec are merged