General: Fix HTML for get_calendar()

The <tfoot> element was allowed to precede the <tbody> element in HTML 5. However, that was changed in HTML 5.1 and <tfoot> must now follow <tbody>.

This removes the tfooot and instead uses a nav element for navigation between months.

Fixes: #39763
Props: isabel_brison, Harry Milatz, audrasjb, abhijitrakas, SergeyBiryukov

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git-svn-id: https://core.svn.wordpress.org/trunk@47023 1a063a9b-81f0-0310-95a4-ce76da25c4cd