v5.2.0 undefined variables for the btn class

[Geuchmann]

Prerequisites

• I have searched for duplicate or closed issues
• I have validated any HTML to avoid common problems
• I have read the contributing guidelines

Describe the issue

The .btn class is using e.g. the --{#prefix}-btn-hover-border-color variable, but it's not declared for it.
It's only decleard over the button-variant mixin for the .btn-#{color} classes.

My expected behavior would be, keep using parent stylings (in this case .btn), if the child stylings (in this case .btn:hover) are missing (or unset because of undeclared variables).
Else we would have a black border in this case, insted of a transparent border (changed behavior compared with v5.0).

What operating system(s) are you seeing the problem on?

Windows

What browser(s) are you seeing the problem on?

No response

What version of Bootstrap are you using?

v5.2.0