change(state): Refactor the naming of note commitment subtrees

[upbqdn]

Motivation

Close #7463.

Solution

#7463 mentions that "We are storing subtree roots and end heights in the subtree indexes, not the subtree trees themselves." I thought of renaming NoteCommitmentSubtree to SubtreeRoot, but there were at least two problems with that:

1. There is also the struct NoteCommitmentSubtreeData, which I wasn't sure how to rename.
2. NoteCommitmentSubtree has a field node, which I had renamed to root, so we would end up with something like subtree_root.root.

Given the issues above, I decided to do a simpler refactor:

1. Rename the generic type Node in NoteCommitmentSubtree to Root.
2. Rename the field node in NoteCommitmentSubtree to root.
3. Rename the field end in NoteCommitmentSubtree to end_height to match the name in the RPC so that the field doesn't need to be renamed for the RPC.

After this refactor, I didn't find any docs that would require changes.

Also, if we ever need to store whole subtrees instead of only roots, we can use the already existing struct NoteCommitmentSubtree, but keep the RPC that currently uses this struct intact because it actually uses NoteCommitmentSubtreeData.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[upbqdn]

I'm not actually sure about the solution in this PR. Do others have suggestions for better solutions?

[arya2]

Looks good, thank you!

[arya2]

Thank you!