[FIX] mail: show tracking value changed messages on different lines

Before this commit, a tracking value message was displayed on a
single line instead of several lines. The parts of the content
of message with tracking values changes were wrapping on multiple
line.

This happens because the content of message had `d-flex`, which
affects notably tracking values that have this DOM shape:
```xml
<div class="o-mail-Message-content d-flex">
    <p>Stage changed</p>
    <ul>
        <li><span/><i/><span/></li>
    </ul>
</div>
```
Because of `d-flex`, `p` and `ul` were next to each other
horizontally, and the `span` in `li` were wrapping to take
responsive layout of `d-flex` into account.

This commit fixes the issue by removing the `d-flex`. This is
undesirable, as the content is HTMl and it should follow the
intended design of the content. `p` and `ul` next to each other,
without specifying custom `display`, should default to 2 `d-block`
next to each other, so they must be on top of each other.

closes odoo/odoo#127320

X-original-commit: c7df8a5fe0c3f785066800e77eef5a46246308dc
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/core/common/message.xml

@@ -73,7 +73,7 @@
                                    'pe-2': env.inChatWindow and !isAlignedRight and composerViewInEditing,
                                    }"
                         >
-                            <div class="o-mail-Message-content position-relative" t-att-class="{ 'd-flex': !state.isEditing, 'w-100': state.isEditing }">
+                            <div class="o-mail-Message-content position-relative" t-att-class="{ 'w-100': state.isEditing }">
                                 <t t-if="message.type !== 'notification' and !message.isTransient and (message.hasTextContent or message.subtypeDescription)">
                                     <LinkPreviewList t-if="!state.isEditing and message.linkPreviewSquash" linkPreviews="message.linkPreviews" deletable="false"/>
                                     <t t-else="">