I18N: Make the minimum input length to start searching posts in the...

I18N: Make the minimum input length to start searching posts in the "Insert/edit link" modal localizable.

Props Toro_Unit, birgire, miyauchi, swissspidy, ocean90.
Fixes #44662.
Built from https://develop.svn.wordpress.org/trunk@46988


git-svn-id: https://core.svn.wordpress.org/trunk@46788 1a063a9b-81f0-0310-95a4-ce76da25c4cd