firmware: update EoL message to reflect reality

src/opnsense/firmware-message

-A placeholder for the 17.7 upgrade message.
+<p>OPNsense 17.1 "<em>Eclectic Eagle</em>" is now end of life. As such
+it will not receive any more updates, but the upgrade to the new 17.7 series is
+seamless and can be performed right here from the GUI by unlocking it below.</p>
+<p> Another method is to import and reinstall using a new installation image,
+which will retain your settings (selecting "Import Configuration"), then
+reformat the disk and apply a clean system (selecting "Guided Installation").</p>
+<p>You can also upgrade via console / SSH by using option 12 from the menu.</p>
+<p>Please backup your configuration, 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.</p>

src/opnsense/mvc/app/views/OPNsense/Core/firmware.volt

@@ -644,7 +644,7 @@ POSSIBILITY OF SUCH DAMAGE.
     <div class="row">
 <?php if (file_exists('/usr/local/opnsense/firmware-message')): ?>
         <div id="firmware-upgrade" style="display:none;"><?= @file_get_contents('/usr/local/opnsense/firmware-upgrade') ?></div>
-        <div id="firmware-message" style="display:none;"><?= @file_get_contents('/usr/local/opnsense/firmware-message') ?></div>
+        <div id="firmware-message" style="display:none;"><?= str_replace(PHP_EOL, ' ', @file_get_contents('/usr/local/opnsense/firmware-message')) ?></div>
         <div class="alert alert-warning" role="alert" style="min-height: 65px;">
             <button class='btn btn-primary pull-right' id="upgrade_maj" style="display:none;"><i id="upgrade_progress_maj" class=""></i> {{ lang._('Upgrade now') }}</button>
             <button class='btn pull-right' id="checkupdate_maj" style="margin-right: 8px;"><i id="checkupdate_progress_maj" class=""></i> {{ lang._('Check for upgrade') }}</button>