Fix libraries feeds date string timezone errors (PP-3938)

[tdilauro]

Description

• Corrects a long-standing typo/error in the date format string used to format the dates in the libraries (full and crawlable) OPDS feeds.

Motivation and Context

On #883, we made our dates timezone aware. The extra "Z%z" at the end of the incorrect format string caused dates to end with the letter "Z" for Zulu/UTC and the timezone +/- offset. This wasn't a problem with naive dates because there was no timezone; however, when we activated timezones, we ended up with dates that ended in "Z+0000". Those dates were not valid ISO 8601.

[Jira PP-3938]

How Has This Been Tested?

• Manually verified that the dates look correct.
• Added new tests to catch any changes to the format string.
• All tests pass locally.
• CI tests and other checks pass.

Checklist

• N/A - I have updated the documentation accordingly.
• All new and existing tests passed.

[jonathangreen]

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