Change implementation details of UpdateManagerUpdateForm to not mention 'disabled'

Problem/Motivation

Follow-up to #3117553: Change UI for /admin/reports/updates/update to not mention "Disabled" modules and themes

dww: The scope creeper in me would want to change variable names and other internal code to match. E.g. stuff like this:

-            $projects['disabled'][$name] = $entry;
+            $projects['uninstalled'][$name] = $entry;

Would that be viable, while we're at it?

xjm:

String changes are also backportable during beta, but form structure changes aren't. So let's keep it to strings.

So here's a follow-up to remove the technical debt of the form structure, variable names, etc.

Proposed resolution

Make sure none of the implementation details of UpdateFormUpdateManager mention 'disabled', either.

Remaining tasks

1. Do it.
2. Make sure we didn't break anything.
3. Reviews/improvements.
4. RTBC.
5. Commit.

User interface changes

None.

API changes

TBD.

Data model changes

None.

Release notes snippet

TBD.