[FIX] odoo_theme: show a cursor when hovering on an image without modal

The images that have the class `o-no-modal` cannot be clicked to open in
a modal, so the cursor should not be changed to a pointer when hovering
on them.

This commit also simplifies the theme's SCSS by replacing the hard-coded
SVG images used in the "Supported versions" page by unicode circles with
a custom styling.

X-original-commit: f648960c78
Part-of: odoo/documentation#2466
Co-authored-by: Elisabeth Dickinson (edi) <edi@odoo.com>

extensions/odoo_theme/static/style.scss

@@ -755,14 +755,18 @@ header {
                     font-weight: $fw_semibold;
                 }
 
-                img:not([alt="green"]):not([alt="orange"]):not([alt="red"]) {
+                img {
                     border: 1px solid $gray-lighter;
                     background: $white;
                     padding: .5rem;
+                    margin-bottom: 1rem;
+                }
+
+                img:not(.o-no-modal) {
                     cursor: pointer;
                 }
 
-                img:not([alt="green"]):not([alt="orange"]):not([alt="red"]), iframe {
+                iframe {
                     margin-bottom: 1rem;
                 }