Customizer: Change "Display Header Text" checkbox label to "Display Site Title...

Customizer: Change "Display Header Text" checkbox label to "Display Site Title and Tagline", to make it clearer that it affects both settings.

Props juanfra, vlad.olaru.
Fixes #31829.
Built from https://develop.svn.wordpress.org/trunk@35899


git-svn-id: https://core.svn.wordpress.org/trunk@35863 1a063a9b-81f0-0310-95a4-ce76da25c4cd