Switch systemd units to use Type=exec instead of Type=oneshot

`oneshot` is designed to be used with RemainAfterExit, but we don't need
that nor use it. After reviewing
<https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html>,
`exec` appears to be the correct type since we want the service to be
running once the binary has started, not once it's finished.

Practically this shouldn't make a big difference except that `systemctl
start <name>` won't wait for the entire command to complete.

Fixes #7349.

securedrop/debian/config/lib/systemd/system/securedrop-cleanup-ossec.service

@@ -2,6 +2,6 @@
 Description=Cleanup OSSEC diff queue
 
 [Service]
-Type=oneshot
+Type=exec
 ExecStart=/usr/bin/securedrop-cleanup-ossec.py
 User=root

securedrop/debian/config/lib/systemd/system/securedrop-reboot-required.service

@@ -2,6 +2,6 @@
 Description=Touch /var/run/reboot-required file
 
 [Service]
-Type=oneshot
+Type=exec
 ExecStart=/usr/bin/touch /var/run/reboot-required
 User=root

securedrop/debian/config/lib/systemd/system/securedrop-remove-packages.service

@@ -4,7 +4,7 @@ ConditionPathExists|=/usr/sbin/ufw
 ConditionPathExists|=/usr/sbin/haveged
 
 [Service]
-Type=oneshot
+Type=exec
 Environment="DEBIAN_FRONTEND=noninteractive"
 ExecStart=/usr/bin/apt-get purge --yes ufw haveged
 User=root