Change Bootstrap Barrio theme version from ~5.0 to ~5.1 for using Bootstrap 4 only

Problem/Motivation

Bootstrap Barrio 5.5.0-beta1 was released 5 Feb 2021 at 01:45 EET
https://www.drupal.org/project/bootstrap_barrio/releases/5.5.0-beta1

Bootstrap Barrio ~5.5 is using Bootstrap 5 now

As Bootstrap v5.0.0-beta1 was released on Dec 7, 2020
https://github.com/twbs/bootstrap/releases/tag/v5.0.0-beta1

Proposed resolution

• Lock Vartheme BS4 to use the Bootstrap Barrio ~5.1
• Not to let the composer require Bootstrap Barrio ~5.0 or ~5.5 when ~5.5 stable go out

Remaining tasks

Lock
Test Build

User interface changes

None

API changes

None

Data model changes

None