Change block entity form handler from 'off_canvas' to 'settings_tray'

Follow up to: #2904134: Settings Tray uses the off-canvas dialog type, but "off_canvas" is not an accurate form plugin name, "settings_tray" is →

Problem/Motivation

Settings Tray uses the form handler key, 'off_canvas', for a block entity form handler. This ties Settings Tray functionality to a particular dialog type. Although Settings Tray uses the off-canvas dialog the need for the different form handler is the Settings Tray functionality of simplifying the Block entity edit from and bringing in related configuration.

It is totally possible the that another module in core or contrib would want to open up another version of the Block entity form in the off-canvas dialog, for instance as a quick way adjust visibility settings.

Proposed resolution

1. Change the form handler key from off_canvas to settings_tray
2. Change the route name from entity.block.off_canvas_form to entity.block.settings_tray_form
3. Change the path to the form from /admin/structure/block/manage/{block}/off-canvas to /admin/structure/block/manage/{block}/settings-tray
4. change the block entity link template from off_canvas-form to settings_tray-form

Remaining tasks

The patch

User interface changes

None

API changes

Internal only

Data model changes

None