[v10.1.x] Dashboard: Fix Variable Dropdown to Enforce Minimum One Selection when 'All' Option is Configured

[grafana-delivery-bot]

Backport 97c3cd1 from #69839

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What is this feature?

When 'Selected(n)' is clicked to deselect all, the behavior is inconsistent; sometimes it defaults back to 'All', and at other times, it leaves an empty field.

I've modified the logic to use the 'Selected(n)' to toggle between standard options and the 'All' option.

Important:

I retained the existing UI and enforced the selection of at least one option, which can be either a standard option or the 'All' option.
The enforcing part is only applied when the "All" option is configured, these changes don't address the enforcement otherwise.

Before: The user was not forced to select an option		After - Enforced the selection of at least one option, which can be either a standard option or the 'All' option.
225590713-b71e2309-ff9c-475b-8c6a-01151814f37b.1.mov		DropdownVariableWithAllOptionBorder.mp4

Who is this feature for?

Everyone

Which issue(s) does this PR fix?:

Fixes #64887

Special notes for your reviewer:

I modified the UI slightly and added a border-bottom, this in my opinion brings more clarity to the users about the special "All" option we allow to configure.

Before		After

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[axelavargas]

Good bot 🤖