dropping support for lower versions

Footnotes are now parsed similar to the corresponding restructuredtext, in that resolution (between definitions and references) and ordering is now deferred to transforms on the doctree.

This allows for the proper interaction with other docutils/sphinx transforms, including those that perform translations.

In addition, an upstream improvement to unreferenced footnote definitions is also added here: sphinx-doc/sphinx#12730, so that unreferenced and duplicate definitions are correctly warned about, e.g.:

```
source/index.md:1: WARNING: Footnote [1] is not referenced. [ref.footnote]
source/index.md:4: WARNING: Duplicate footnote definition found for label: 'a' [ref.footnote]
```

It is of note that warnings for references with no corresponding definitions are deferred to docutils to handle, e.g. for `[^a]` with no definition:

```
source/index.md:1: ERROR: Too many autonumbered footnote references: only 0 corresponding footnotes available. [docutils]
source/index.md:1: ERROR: Unknown target name: "a". [docutils]
```

These warning messages are a little obscure, and it would be ideal that one clear warning was emitted for the issue.
However, it is non-trivial in this extension; to both suppress the existing warnings, and then replace them with a better one,
so for now we don't do it here, and ideally this would be improved upstream in docutils.