Customize: improve consistency of the "No themes found" strings

[afercia]

See #37661.

For consistency, a couple strings introduced in [41648]:

No themes found. Try a different search.
No themes found. Try a different search, or %s.

Should be updated to the new wording introduced in [41608] which added the word "query".

All similar strings in WordPress are now, for example:
No themes found. Try a different search query.
or, for plugins,
No plugins found. Try a different search query.

[melchoyce]

Could we remove "query" from the other strings? It's a pretty unfriendly word.

[celloexpressions]

Replying to melchoyce:

Could we remove "query" from the other strings? It's a pretty unfriendly word.

Agreed. Or, find a less technical-sounding term to use here (for English, at least).

[Presskopp]

I think 'search term' could be useful here..

[afercia]

Could we remove "query" from the other strings? It's a pretty unfriendly word.

Then I think #37430 should be reopened for discussion, see https://core.trac.wordpress.org/ticket/37430#comment:16

[swissspidy]

As there wasn't really a discussion on that, let's just remove "query". Less new strings for translators!

[afercia]

42231.diff​ partially reverts [41608] to restore the original strings without the word query.

[afercia]

In 41878:

Administration: Improve consistency of the "not found" strings.

Partially reverts [41608] restoring the original strings without the word query.

See #37430.
Fixes #42231.