Improve visual separation of select optgroup headers

[archilex]

Description

When using optgroups with selects the headers look too much like clickable options causing confusion for users. This PR uses darker text to indicate they are not clickabled, but also semibold to indicate it's not just a disabled option.

.fi-dropdown-header was targeted from the select.css class instead of directly inheader.css because .fi-dropdown-header is also used for the dropdown blade component and those styles shouldn't be affected.

Visual changes

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.