[1.12.0 backport] Use polkadot-ckb-merkle-mountain-range dependency (…

…#5897)

# Description

Polkadot SDK v1.12.0 `mmr-lib` dependency of the `sp-mmr-primitives`
crate is set to the package `ckb-merkle-mountain-range` and pinned to
the master branch of
[this](https://github.com/paritytech/merkle-mountain-range) repo. The
package name in that branch has changed to
`polkadot-ckb-merkle-mountain-range`, which causes v1.12.0 to not
compile.

This PR does a simple backport to utilize the
`polkadot-ckb-merkle-mountain-range` published package instead, and pins
it to version 0.7.0, as was done in #4562