TinyMCE: Fix the keyboard shortcut (Alt+Shift+H) to not open the default Block...

TinyMCE: Fix the keyboard shortcut (Alt+Shift+H) to not open the default Block Editor help modal and change the title of the help modal to "Classic Block Keyboard Shortcuts" in the Classic BLock. Also remove Alt+Shift+Z in the Classic Block as it conflicts with the Block Editor.

Props azaozz.
Merges [43915] to trunk.
Fixes #45365.
Built from https://develop.svn.wordpress.org/trunk@44257


git-svn-id: https://core.svn.wordpress.org/trunk@44087 1a063a9b-81f0-0310-95a4-ce76da25c4cd