Make -checkpoints argument no longer debug-only #28524

A small step towards dropping checkpoints entirely, make the option to disable them a bit easier to find.

bitcoind --help
...
  -checkpoints
       Enable rejection of any forks from the known historical chain until
       block 295000 (default: 1)

Before this PR this required bitcoind --help-debug.

A potential next step could be to change the default to 0 in a future release, then drop the checkpoints in the next release (#28524) and then drop all the checkpoint code one release later.

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make the option to disable them a bit easier to find.

Not sure. If we aren't completely convinced about actually dropping them yet, I'm not sure that making the option more apparent to users is the right thing to do.

That makes sense. Marking this as draft until we feel confident enough to advertise this option. I've been running it for half a year :-)

Closing for now. This is a simple change, but doesn't make sense yet, and can easily be reopened / redone once there is a clear path forward for #25725.