Fix space between popover's arrow and triggering element

This PR is a proposal to:

• Fix the space between the popover's arrow and the triggering element.
• Add --bs-popover-inner-border-radius as it is done for the dropdowns with --bs-dropdown-inner-border-radius

(Note: I can split the commits if needed)

Regarding the fix proposed here.

In https://github.com/twbs/bootstrap/commit/2e75ec3c41a54865caa48f46485a1c90b12b0de6 (and so in the main branch) we can observe this kind of modifications:

- bottom: subtract(-$popover-arrow-height, $popover-border-width);
+ bottom: calc((var(--#{$variable-prefix}popover-arrow-height)) * -1); // stylelint-disable-line function-disallowed-list

The substraction does not appear in the new calculation. So the rendering is slightly different:

Before

Preview: https://getbootstrap.com/docs/5.1/components/popovers/#four-directions

With the error (main branch)

Preview: https://twbs-bootstrap.netlify.app/docs/5.1/components/popovers/#four-directions

There is an extra spacing between the button and the arrow:

With the fix proposed in this PR

Preview: https://deploy-preview-35976--twbs-bootstrap.netlify.app/docs/5.1/components/popovers/#four-directions