asset: migrate stylesheets from Sass to CSS (ranguba#37)

GitHub: ref rangubaGH-35

This pull request migrates the stylesheets from Sass (SCSS) to plain
CSS.
The primary reason for this migration is that `node-sass`, a dependency
for Sass, has been deprecated.
By transitioning to CSS, we simplify our dependencies and avoid using
deprecated tools.

## Changes
- Renamed search.scss to search.css.
- Updated comments format from Sass to CSS style.

## Migration steps

- [x] Change the file extension <- This PR is here.
- [ ] Remove sass-rails

app/assets/stylesheets/search.scss → app/assets/stylesheets/search.css

@@ -1,6 +1,7 @@
-// Place all the styles related to the posts controller here.
-// They will automatically be included in application.css.
-// You can use Sass (SCSS) here: http://sass-lang.com/
+/*
+ * Place all the styles related to the posts controller here.
+ * They will automatically be included in application.css.
+ */
 
 /* layout */
 .content {