Technical debt: `sass` pinned due to deprecation warnings with bootstrap

[SamuelT-Beslogic]

We currently pin "sass": "1.77.6"

See for potential temporary solutions (which should be clearly documented with an action once fixed in bootstrap). Or just wait for those to be resolved:

• dart-sass 1.80.0+ throwing a lot of deprecations twbs/bootstrap#40962 (comment)
• New deprecations in Sass 1.79 twbs/bootstrap#40849

[SamuelT-Beslogic]

In #359 I changed the pins to:

• "bootstrap": "^5.3.4"
• "sass": "<1.79"

See https://github.com/orgs/twbs/discussions/41370