Upgrade/Install: Use ARIA button class on plugin and theme auto-updates action links

According to the [accesability guidelines](https://developer.wordpress.org/coding-standards/wordpress-coding-standards/accessibility/#semantics-for-controls), the control should be a link when JavaScript is not available and a button the rest of the time.

In addition, handlers were added for spacebar usage, and some changes to the a11y speak verbiage.

Fixes #50516.
Props ryokuhi, audrasjb, afercia, whyisjake/



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