[BITV]: NcSelect improvements

[JuliaKirschenheuter]

This is accessibility issue.

There are several point to improve:

1. Preselected time zone (Europe - Amsterdam) have not enough contrast and seems like
   

2. Group item like "Africa" have not enough contrast and is not recognizable even if it is disabled item.
   

3. Aria-label of combobox is "search for option" and have to be instead "Search for time zone"
   

4. strong timezone have to be a label for Select
   

[susnux]

1. Preselected time zone (Europe - Amsterdam) have not enough contrast and seems like

• done with fix(NcSelect): Ensure selected option has enough contrast when the menu is opened nextcloud-libraries/nextcloud-vue#4733

2. Group item like "Africa" have not enough contrast and is not recognizable even if it is disabled item.

• done with fix(theming): Ensure all text colors have enough contrast for accessibility #40773

3. Aria-label of combobox is "search for option" and have to be instead "Search for time zone"

• TBD

4. `strong` timezone have to be a label for Select

• done with fix(dav): Add ID / label to timezone picker in availablility settings #41285