Add timezone format to last checked time

[mkaz]

This adds the additional format T to the date format that will also display the timezone abbreviation. If timezone is set to UTC+0 it will show GMT+0000, if timezone set to a name, it will use the name abbreviation, for example PDT for America/Los_Angeles.

Trac ticket: https://core.trac.wordpress.org/ticket/53554

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[hellofromtonya]

Committed via https://core.trac.wordpress.org/changeset/52203.