fix deprecated sass

[cleydyr]

The following warnings appear in the log:

[INFO] --- dart-sass-maven-plugin:0.3.0:compile-sass (default) @ biblivre ---
Deprecation Warning: Using / for division outside of calc() is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div(-$content-width, 2) or calc(-$content-width / 2)

More info and automated migrator: https://sass-lang.com/d/slash-div

    ╷
235 │     margin-left: - $content-width / 2;
    │                  ^^^^^^^^^^^^^^^^^^^^
    ╵
    src/main/sass/interface/_menu.scss 235:15  @import
    src/main/sass/biblivre.core.scss 32:9      root stylesheet

Deprecation Warning: Using / for division outside of calc() is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div(-$content-width, 2) or calc(-$content-width / 2)

More info and automated migrator: https://sass-lang.com/d/slash-div

    ╷
279 │     margin-left: - $content-width / 2;
    │                  ^^^^^^^^^^^^^^^^^^^^
    ╵
    src/main/sass/interface/_menu.scss 279:15  @import
    src/main/sass/biblivre.core.scss 32:9      root stylesheet