Refactor changelog to use new \ChangelogRef

[BryantLam]

This PR primarily refactors the references in the changelog entries to reduce inconsistencies between entries and to make it easier to later alter its format if desired.

I highly recommend reading the changes in each commit rather than all changes together. This is because eff0264 adds new deprecation-specific labels and then applies them into the changelog — in my opinion this is the only non-trivial change to review and I don't want it to get muddied by the mostly mechanical conversion done in e3dd9d1 which are the bulk of the changes.

Notes:

1. eff0264 tries to add references to each deprecated item. Turns out some of the older changelog entries were somewhat ambiguously phrased, so these references should definitely help disambiguate the changes in each of the affected changelog entries.
2. e3dd9d1
   • Each \ChangelogRef ends in a % sigil as a visual reminder to not add a period.
   • As a restriction, I didn't reword any of the existing entries to make this commit as mechanical as possible.
3. \ChangelogRef is implemented using package cleveref. By default, references are sort&compress. For our purposes, sort is fine. Compress, however, means that many-reference entries are compressed into a range instead; e.g., See Sections 1, 2, 3, 4 and 5. is converted into See Sections 1 to 5.. If this behavior is not desired, I can easily change it.
   • There's a bypass. One of the changelog entries uses double-commas ,, because it references tables and tables are spread throughout the document; its references shouldn't be compressed.
4. cleveref doesn't use the Oxford comma (:cry:). I left it this way, but if we strongly oppose to its non-use, I can change that too.

[jdinan]

Looks great -- thanks for these edits! A few minor nitpicks and also wondering whether we want to add back some changelog references that were substituted with references to the deprecation annex.