[5.4] Passwort reset request: fix legend

[chmst]

Pull Request for Issue #46446 .
Replaces #46451

Summary of Changes

This PR sepaprates Legend of the fieldset and label of a field in all pages for password reset. (request, confirm, complete)

Testing Instructions

I a frontend login form click the "forgot password" link.

Actual result BEFORE applying this Pull Request

The label of the fieldset is used for a legend. If there is no label, there would not be a legend at all, which is an a11y issue.
A legend is not an instruction but summarises the content of a fieldset.
This PR adds a legend for the fieldset, if there is a label or not.

### Expected result AFTER applying this Pull Request

Note:
If this is accepted, same change is needed for similiar pages in the users component.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[Sieger66]

Same can you do at:
https://github.com/joomla/joomla-cms/blob/5.4.0/components/com_users/tmpl/remind/default.php#L35-L37
https://github.com/joomla/joomla-cms/blob/5.4.0/components/com_users/forms/remind.xml#L7
for the Login reminder page.