Update sass when bootstrap uses the new sass module system

[RandomTannenbaum]

While upgrading Rails to v8 I pinned the sass version to 1.77.6 for now, because after this version we get deprecation warnings from bootstrap, because it doesn't use the new sass module system. Our app currently also doesn't do this, so this should be done in the same step to also remove these deprecation warnings. Also as of sass version 1.80.0, imports with @import are deprecated. This also produces a lot of deprecation warnings originating in bootstrap.

References:

• https://github.com/twbs/bootstrap/issues/29853
• https://stackoverflow.com/questions/67675422/deprecation-warning-in-bootstrap-scss
• https://sass-lang.com/documentation/at-rules/import/

ToDo

• [ ] Update bootstrap and sass when bootstrap uses the new sass module system
• [ ] Update our scss to files to also use the new module system
• [ ] Fix any other deprecations originating from the updates