doc(plugin.xml): add CocoaPods deprecation notice to podspec section

[GitToTheHub]

Platforms affected

Motivation and Context

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[dpogue]

Minor correction for date format, but this otherwise looks good

[GitToTheHub]

Ok. Is there a reason why 2. should be written as 2<sup>nd</sup>? Is this something semantic or just proper English?

[dpogue]

English would never use "2.", because the period looks like the end of the sentence.
"2^nd" is most correct, but "2" on its own would also be considered acceptable.

[GitToTheHub]

Ok thanks for the explanation, I didn't know. Always nice to learn new things :)