Fix Olivero's drop-button style to conform with new form styles

#4, Looks fine to me. RTBC +1

Applied patch #4 and it works fine.

Before patch:

After patch:

RTBC +1

@mherchel Please review the patch which addresses the single variant. Attached screenshots.

Updated patch

Looks and works great. Marking RTBC!

Patch #15 look good and works great. Marking this RTBC!

Works good! Moving to RTBC +1!

Few things I noticed while testing this:

1. 
   The text on the button appears to be off-center.
2. The focus effect is inconsistent between the actions and the toggle button. On the actions the outline is rendered inside the button but on the toggle it's rendered outside.

Hi @mherchel
#19 patch Works good for me! Moving to RTBC +1!

The button text is now aligned, and it looks great:

I think there is a grey line appearing under the dropbutton in the down state that is not meant to be there. See:

This extra line is cause by the border: solid 1px transparent; in

.dropbutton-action:first-child {
    border: solid 1px transparent;
    border-right: solid 2px #fff; /* LTR */
    border-radius: 0.1875rem 0 0 0.1875rem; /* LTR */
  }

without that line the drop buttom looks way sharper.

Address #24.

Verified and tested patch #27.
Patch applied successfully and looks good to me.

Testing Steps:
# Set the Olivero theme from Appearance.
# Goto: /admin/structure/types/manage/article/fields
# User should see dropdown-button style to conform with new form styles.

Looks good to me.
Can be a move to RTBC
Please refer attached screenshots for Before and After patch.

Restoring status after HEAD was broken.

+++ b/core/themes/olivero/css/components/dropbutton.pcss.css
@@ -1,76 +1,155 @@
+  z-index: 50; /* Ensure this appears above all other dropbuttons. */

Based on https://www.drupal.org/docs/theming-drupal/z-indexes-in-drupal-8, shouldn't this be 100?

1. 
   Long texts are not legible in browsers that don't support max-content
2. 
   Dropbuttons with just a single item are missing border radius on the right

Hi @mherchel the second point is still not fixed from #36 by @lauriii

1. Still we missed the border-radius
Attaching SS:

2. When test is long
SS after patch for long text (without limitting the wrapper width).

3. Here we couldn't see the separation between the two dropdowns
SS after patch for long text (with limiting the wrapper width).

All these text is edited through browser dev-tools.

Attaching the SS of long dropdown item before patch (might be helpful for finding the 3rd point):

Hi @mherchel thanks for the patch, now it's working as expected.

Attaching screenshot after patch #40

This was remaining task on this issue , now it's get resolved. So am moving the issue to RTBC.

Committed 61ebd08 and pushed to 9.3.x. Thanks!