Add WantedBy=sockets.target to mariadb.socket systemd

per https://lists.mariadb.org/hyperkitty/list/discuss@lists.mariadb.org/thread/U5ULJ6D2SJDKA3JR77755XQHX3BFLKWM/

I wanted to make use of socket based activation since I won't need frequent
access to my databases.

I was reading the mariadb systemd documentation at:
https://mariadb.com/docs/server/server-management/starting-and-stopping-mariadb/systemd#using-systemd-socket-activation
This explains that mariadb supports socket activation which is great, but mariadb.socket
and others are missing their [Install] section.

I've solved this temporarily by including a drop-in file:
/etc/systemd/system/mariadb.socket.d/00-install.conf
```
[Install]
WantedBy=sockets.target
```

However this seems like a huge oversight to me. So much that I'm confused if this
is in fact an intentional decision. Socket activation was introduced in MDEV-5536
over 4 years ago, and in that time no changes have been made. I was not able to
find any mention of a lack of [Install] in documentation of git commit history.
In fact the documentation I list above explicitly mentions installing the socket
using `systemctl enable mariadb.socket` which right now throws an error.

I'd appreciate some clarification on this.