Rename "Configure Shortcuts" to "Configure Keyboard Shortcuts"

Summary:
This is a bit clearer, and reinforces the keyboard icon used for the menu item.

BUG: 39488
FIXED-IN: 5.66

If accepted, will wait until after tagging to land it so as not to break the string freeze.

Test Plan:

Reviewers: VDG, ndavis

Reviewed By: VDG, ndavis

Subscribers: ltoscano, ndavis, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25793