firmware: remove EOL message, we are on 16.1 now

src/opnsense/firmware-message

-<p>Hello trusty friend,</p>
-<p>As with all things in life, the 15.7 series of OPNsense is nearing its
-inevitable end. As such it will not receive any more updates, but the
-upgrade to the new 16.1 series is seamless, except for the following
-points:</p>
-<p><ul><li>The FreeBSD version changes from 10.1 to 10.2, mainly for driver
-updates and general sanity. <strong>If you're running Hyper-V, your installed
-disk may change from <em>/dev/ada0</em> to <em>/dev/da0</em> and the system
-will not boot as a consequence</strong>. You can fix this by manually editing
-<em>/etc/fstab</em> before performing the major upgrade. A reinstall using the
-import configuration and quick/easy install will work just as well.</li>
-<li>The captive portal implementation has been completely rewritten.
-<strong>Your old setup won't be converted and thus will not come back
-up</strong>. Reconfiguring the captive portal, however, will be relatively
-easy.</li>
-<li>Due to kernel interface changes going from 10.1 to 10.2 <strong>plugins
-or custom-built kernel modules may stop working</strong>. Reinstalling the
-offending packages from the firmware pages or recompiling custom additions
-against the <em>stable/16.1</em> source branch will resolve this
-problem.</li></ul></p>
-<p>Please heed these points carefully before upgrading. Backup your configs,
-preview the new version via the live CD or in a virtual machine. Create
-snapshots. If all else fails, report back
-<a href="https://forum.opnsense.org/" target="_blank">in the forums</a> for
-assistance. You don't have to do this on your own. :)</p>
-<p><em>Brave Badger</em>, you've served us well.</p>
-<p>See you on the other side,<br>Your OPNsense team</p>