Recent Posts Widget: Use `ob_end_flush()` instead of `ob_flush()`.

`ob_end_flush()` flushes the output buffer *and* turns output buffering off, same as `ob_get_flush()`.

props m_i_n.
see #28009 for trunk.

Built from https://develop.svn.wordpress.org/trunk@28195


git-svn-id: http://core.svn.wordpress.org/trunk@28025 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Force users to choose a nav menu in the custom nav menu widget, for a better customizer UX

Before, they had to make a dummy change to get it to render. Now they
are made to choose a nav menu from the dropdown, which feels more
natural.

fixes #27878 for trunk. props westonruter
Built from https://develop.svn.wordpress.org/trunk@28197


git-svn-id: http://core.svn.wordpress.org/trunk@28027 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Use '&mdash; Select &mdash;' instead of '-- Select --' for nav menus. Looks nicer.

see #27878
Built from https://develop.svn.wordpress.org/trunk@28205


git-svn-id: http://core.svn.wordpress.org/trunk@28035 1a063a9b-81f0-0310-95a4-ce76da25c4cd