[FIX] mail: dialog properly manage overflow for long content

Before this commit, when content of dialog was long, content
overflow but was not scrollable, which makes bottom unreachable,
which is especially a problem when buttons are at the bottom.

This problem solves the issue by adding classname `modal` to
dialog. This classname puts proper handling of overflow of
dialog and its content.

Also remove `align-items-center` so that top doesn't overflow
in isolation from the rest of content, which made top unreachable.

Task-2973866

closes odoo/odoo#99865

X-original-commit: 9f4495aa065da5e94d6ccd007c9f26b7d755f140
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

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 fixed-top bottom-0 d-flex justify-content-center align-items-center" t-attf-class="{{ className }}" t-att-style="dialog.style" t-ref="root">
+            <div class="o_Dialog modal fixed-top bottom-0 d-flex justify-content-center" t-attf-class="{{ className }}" t-att-style="dialog.style" t-ref="root">
                 <t
                     t-component="constructor.components[dialog.componentName]"
                     t-props="{