feat(Spinner): make the animation default to border (#6468)

* make the spinner animation default to border

There's no good reason to always type out the animation for the spinner. There are only two choices, and the "border" option is the obvious choice as a default.

This should be a non-breaking change.

* use default prop for spinner animation parameter

src/Spinner.tsx

@@ -9,7 +9,7 @@ import { Variant } from './types';
 export interface SpinnerProps
   extends React.HTMLAttributes<HTMLElement>,
     BsPrefixProps {
-  animation: 'border' | 'grow';
+  animation?: 'border' | 'grow';
   size?: 'sm';
   variant?: Variant;
 }
@@ -64,7 +64,7 @@ const Spinner: BsPrefixRefForwardingComponent<'div', SpinnerProps> =
       {
         bsPrefix,
         variant,
-        animation,
+        animation = 'border',
         size,
         // Need to define the default "as" during prop destructuring to be compatible with styled-components github.com/react-bootstrap/react-bootstrap/issues/3595
         as: Component = 'div',
@@ -75,7 +75,6 @@ const Spinner: BsPrefixRefForwardingComponent<'div', SpinnerProps> =
     ) => {
       bsPrefix = useBootstrapPrefix(bsPrefix, 'spinner');
       const bsSpinnerPrefix = `${bsPrefix}-${animation}`;
-
       return (
         <Component
           ref={ref}