Update Bootstrap framework

[mmcinnestaylor]

https://getbootstrap.com/docs/5.0/migration/

1. To begin the migration, replace the framework files located at src/assets/vendors/bootstrap with their v5.3.x correlates.
2. Next, ensure the framework is being properly called from the base template (which nearly all project templates extend).
   • src/templates/base.html
3. Update the link in CONTRIBUTING.md to correspond to the updated documentation.