Change UI for /admin/reports/updates/update to not mention "Disabled" modules and themes

Problem/Motivation

Discovered while working on #3113992: The 'Update' page has no idea that some updates are incompatible

Drupal 8 no longer has the notion of "disabled modules". Either they're installed and enabled, or completely uninstalled. There's no such thing as an installed + disabled module. For example, the 'Available updates' listing report (at /admin/reports/updates) looks like this:

However, the 'Update' page at /admin/reports/updates/update was never modified to account for this change in the rest of core's extension handling. If you enable the "[x] Check for updates of uninstalled modules and themes" checkbox at /admin/reports/updates/settings and then visit the 'Update' form, you currently see this:

Proposed resolution

Modify the 'Update' form UI to match the 'Available updates' report to talk about "Uninstalled" not "Disabled". Also change the table heading from "Installed version" to become "Local version".

Remaining tasks

1. Decide exactly what to do
2. Do it
3. Wait for #3113992: The 'Update' page has no idea that some updates are incompatible to land so we have a test class to extend to test this
4. Reviews
5. RTBC
6. Commit

User interface changes

After (patch #11)

p.s. Yes, this still needs help. See #3116542: Make column widths consistent on the Update form for more.

API changes

None.

Data model changes

None.

Release notes snippet

TBD, probably not.