Updates Screen: remove the "check again" `button` CSS class for better visual hierarchy

[audrasjb]

In the core-auto-updates Slack channel, @karmatosed pointed out that the "Check again" button would benefits to looks like a link and not a button. Indeed, there are many buttons on this screen and this one doesn't have the same place ion the screen hierarchy than the other ones. Moreover, this button is actually… a link. So there is no accessibility issue to make it looks like a link :)

[karmatosed]

Thanks for making this ticket @audrasjb, it's a great little benefit here to ease the complexity of the screen with numerous buttons.

[audrasjb]

Upgrade/Install: Replace "Check again" button style with normal link style for better visual hierarchy.

[audrasjb]

Before patch

[audrasjb]

After patch

[audrasjb]

Pinging @karmatosed for a quick design review :)

[karmatosed]

Wow, that was fast! This is awesome and quick design review is to ship it!

[helen]

After patch, no updates available

[helen]

I attached a screenshot with this patch when no updates are available, just for reference. I think it does still look better to not have two secondary buttons, perhaps especially without a primary button present.

I also traced back through history and this was always a button, so no need to worry that we're undoing a previous decision to make this a button. Will handle commit momentarily.

[helen]

In 49150:

Updates: Reduce secondary buttons for less visual complexity.

Props audrasjb, karmatosed.
Fixes #51523.

[afercia]

Totally in favor of making "Check Again" look like a link. Just a quick question: since this is now a link that looks like a link, does it still make sense to use title case? A textual link using title case seems a bit odd to me. I'd tend to think it should now use sentence case. Maybe also consider to add a period after the link.

[SergeyBiryukov]

In 49471:

Upgrade/Install: Switch "Check again" link on WordPress Updates screen to sentence case.

This brings some consistency with other text links on the screen, e.g. "View version details" for plugins or themes.

Follow-up to [49150].

Props afercia.
See #51523.