Update FTP credentials form design

[swissspidy]

As reported by @melchoyce in ​https://github.com/obenland/shiny-updates/issues/100, the filesystem credentials dialog needs some love to improve accessibility and the overall design.

Here's how it could look like: ​https://cloudup.com/cnvj5NqMefR

Her notes:

1. Update modal style to bring more in-line with other wp-admin modals — like including an "x" for closing (even though there's also a cancel button)
2. Make "Proceed" primary
3. Also consider changing "proceed" to something more descriptive — maybe connect? Proceed might be okay, it's just such a weird-sounding word. Maybe switch the "proceed" in both the description and the button to "continue."
4. "This password will not be..." could stand to be decreased in size just a little and made a lighter grey, to match descriptions on the settings pages. At the very least, the line-height should decrease.
5. "Authentication Keys" is at the same level of hierarchy as the field labels. It should probably be bigger, since it's a header above the labels.
6. Additionally: the modal is really tall for mockup purposes, but would ideally be whatever height we normally max modals out at. Any content not visible because of height would be scrollable in the modal, like we do in others.

See also: #34376

[afercia]

See also #37484