[FIX] mail: improve arrow buttons label readability

This commit improves the readability of the label inside an arrow button.

Prior to this commit, the label of an arrow button was styled with a
`opacity-50` class. This would render well visually, but does not provide
sufficient contrast.

To improve the readability of this label, we simply set a `text-muted`
class to it, which will still apply an opacity, but more subtle, making
our label readable.

task-3575154

closes odoo/odoo#144110

X-original-commit: a008b0f8e794a7c00719838c6f2a05b97b021262
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/views/fields/statusbar_duration/statusbar_duration_field.xml

@@ -5,7 +5,7 @@
     <t t-name="mail.StatusBarDurationField.Dropdown" t-inherit="web.StatusBarField.Dropdown" t-inherit-mode="primary">
         <xpath expr="//*[@t-esc='item.label']" position="inside">
             <span t-esc="item.label" />
-            <span t-if="item.shortTimeInStage" t-att-title="item.fullTimeInStage" class="ms-2 opacity-50 small">
+            <span t-if="item.shortTimeInStage" t-att-title="item.fullTimeInStage" class="ms-2 text-muted small">
                 <t t-esc="item.shortTimeInStage" />
             </span>
         </xpath>
@@ -21,7 +21,7 @@
 
         <xpath expr="//*[@t-esc='item.label']" position="inside">
             <span t-esc="item.label" />
-            <span t-if="item.shortTimeInStage" t-att-title="item.fullTimeInStage" class="ms-2 opacity-50 small">
+            <span t-if="item.shortTimeInStage" t-att-title="item.fullTimeInStage" class="ms-2 text-muted small">
                 <t t-esc="item.shortTimeInStage" />
             </span>
         </xpath>