feat: quiet sass deprecation warnings

[elasticspoon]

We have a lot of these in the app atm:

Deprecation Warning: Sass's behavior for declarations that appear after nested
rules will be changing to match the behavior specified by CSS in an upcoming
version. To keep the existing behavior, move the declaration above the nested
rule. To opt into the new behavior, wrap the declaration in `& {}`.

More info: https://sass-lang.com/d/mixed-decls

    ┌──> node_modules/bootstrap/scss/_type.scss
40  │       font-weight: $display-font-weight;
    │       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ declaration
    ╵
    ┌──> node_modules/bootstrap/scss/vendor/_rfs.scss
136 │ ┌     @media (#{$rfs-mq-property-width}: #{$rfs-mq-value}) {
137 │ │       @content;
138 │ │     }
    │ └─── nested rule
    ╵
    node_modules/bootstrap/scss/_type.scss 40:5  @import
    bootstrap/scss/bootstrap.scss 17:9           @use
    app/assets/stylesheets/application.scss 3:1  root stylesheet

This is because sass added those in 1.77.7 but bootstrap is not up to date to that yet.

Thus, we will pin sass to 1.77.6 until bootstrap 5.3.4 is out to silence sass deprecation warnings.

See this issue on bootstrap for details: twbs/bootstrap#40621