[REF] mail: `dialog` scss revamp

This commit replaces raw SCSS with default Bootstrap classes, in
order to reduce code lines and to increase performance.

Task-2812497
Part of the overall v16 SCSS optimization/restyle, task-2704984

closes odoo/odoo#87796

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/components/dialog/dialog.scss

@@ -2,18 +2,6 @@
 // Layout
 // ------------------------------------------------------------------
 
-.o_Dialog {
-    position: absolute;
-    top: 0;
-    bottom: 0;
-    left: 0;
-    right: 0;
-    display: flex;
-    align-items: center;
-    justify-content: center;
-    z-index: $zindex-modal;
-}
-
 .o_Dialog_componentLargeSize {
     min-width: Min(95vw, $o-modal-lg);
     max-width: 95vw;

addons/mail/static/src/components/dialog/dialog.xml

@@ -3,7 +3,7 @@
 
     <t t-name="mail.Dialog" owl="1">
         <t t-if="dialog">
-            <div class="o_Dialog" t-attf-class="{{ className }}" t-att-style="dialog.style" t-ref="root">
+            <div class="o_Dialog fixed-top bottom-0 d-flex" t-attf-class="{{ className }}" t-att-style="dialog.style" t-ref="root">
                 <t
                     t-component="constructor.components[dialog.componentName]"
                     t-props="{