COUNTER-SET for alphabetical ordered lists ignored for utf-8 exporter

["Tom Alexander" <tom@fizz.buzz>]

It seems that COUNTER-SET[1] is not being honored when exporting to utf-8 for alphabetical lists even though it is honored for numeric lists. When exporting to html, COUNTER-SET is honored for both.

Test document:
```
# An ordered list starting at 3
1. [@3] foo


# An ordered list starting at 1
m. bar


# An ordered list starting at 11
m. [@k] baz
```

Launching emacs with: (Setting org-list-allow-alphabetical is necessary or else the alphabetical lists will become paragraphs)
```
emacs -q --eval '(setq org-list-allow-alphabetical t)' /tmp/test.org
```

When exporting to html you get (edited to remove whitespace for clarity):
```
<ol class="org-ol"><li value="3">foo</li></ol>
<ol class="org-ol"><li>bar</li></ol>
<ol class="org-ol"><li value="11">baz</li></ol>
```

But when exporting to utf-8 you get: (whitespace removed again)
```
3. foo
m. bar
m. baz
```

Whereas I would expect:  (whitespace removed again)
```
3. foo
m. bar
k. baz
```

On a slightly related note: it seems the COUNTER-SET[1] allows single-letter values even when org-list-allow-alphabetical is nil. I don't think that is going to hurt anyone but I figured I should mention it in case its a bug (test doc: `1. [@k] foo` is a plain list starting at 11 even when org-list-allow-alphabetical is nil).

[1] https://orgmode.org/worg/org-syntax.html#Items

Emacs 29.1, Org-mode version 9.7-pre (release_9.6.8-781-gc70354)

--
Tom Alexander
pgp: https://fizz.buzz/pgp.asc