Update extensions page title to be more clear

Problem/Motivation

With the extensions module enabled, we get two new tabs in the extend section: 'Update' and 'Update Extensions'. It's not clear from this that one is for core and one is for contrib, so I think we can try to clarify this slightly by renaming them. I don't want to try to solve the UX of this section here, so am proposing a minor tweak only.

Also, the 'Update Extensions' page currently has the title 'Automatic Updates Form'. Guessing this is just an oversight.

Before:

Proposed resolution

Rename the tabs and the page. Also remove the colons from the table headers, and use sentence case, to conform with the standard style.

After:

Remaining tasks

Create an MR with the changes
Update tests if needed?
Review
Merge

User interface changes

As noted.