This PR will make omni-node dev-mode once again compatible with older
runtimes.

The changes introduced in
https://github.com/paritytech/polkadot-sdk/pull/6825 changed constraints
that are enforced in the runtime. For normal chains this should work
fine, since we have real parameters there, like relay chain slots and
parachain slots.

For these manual seal parameters we need to respect the constraints,
while faking all the parameters. This PR should fix manual seal in
omni-node to work with runtime build before and after
https://github.com/paritytech/polkadot-sdk/pull/6825 (I tested that).

In the future, we should look into improving the parameterization here,
possibly by introducing proper aura pre-digests so that the parachain
slot moves forward. This will require quite a bit of refactoring on the
manual seal node side however. Issue:
https://github.com/paritytech/polkadot-sdk/issues/7453

Also, the dev chain spec in parachain template is updated. This m...