Use the ".. deprecated::" directive for deprecations

[OskarStark]

Refs #11154

Todo:

• We need no ensure the correct html markup
• @symfony/docs-team please review the new documentation about "how/when to use this new directive"

[wouterj]

Approved! And added "on hold", as we first have to make sure @javiereguiluz adds nice styling for this on symfony.com. The deprecated directive is rendered the same way as the versionadded (including the .versionmodified class), except that it's main class is deprecated. So it should be fairly easy to reuse the versionadded styles and only modify the color (i.e. some blackish)

[javiereguiluz]

There is a pending PR on symfony.com private repo to add the styles:

Before

After

And in dark mode:

[wouterj]

Nice!

[OskarStark]

Wow this looks amazing 🥰👍🏻

[javiereguiluz]

The style changes were merged on symfony.com, so we can now merge this. Thanks Oskar!