Update Bootstrap to match redesign colors and update `button` styles

Created by: umpox

This PR:

• Enables the updated Bootstrap colors for the design refresh
• Updates button variants to match design refresh
• Updates text- and alert variants to roughly match design refresh. There may be further work here to implement more specific parts of these styles

Closes https://github.com/sourcegraph/sourcegraph/issues/20394 Closes https://github.com/sourcegraph/sourcegraph/issues/19969 Closes https://github.com/sourcegraph/sourcegraph/issues/20016 Closes https://github.com/sourcegraph/sourcegraph/issues/20092 Closes https://github.com/sourcegraph/sourcegraph/issues/20172