chore: update slack link

The old link expired but the website still points to it. Let's update it
to the new one.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

index.html

@@ -129,7 +129,7 @@
             <span>Join our GitHub Community</span>
           </a>
           <a
-            href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-2tc0mfxpc-tU~HYAYSzLDl5XGGJU3YIg"
+            href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-3v81btcpe-usCf8Qk7k1gUlSAEKKdYMg"
             class="css-button sta-hoverOFF"
           >
             <i class="icon-slack" aria-hidden="true"></i>
@@ -324,7 +324,7 @@
               </a>
               . You can chat with others in the community on the
               <a
-                href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-2tc0mfxpc-tU~HYAYSzLDl5XGGJU3YIg"
+                href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-3v81btcpe-usCf8Qk7k1gUlSAEKKdYMg"
               >
                 Firecracker Slack workspace
               </a>
@@ -387,7 +387,7 @@
             <span>Join our GitHub Community</span>
           </a>
           <a
-            href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-2tc0mfxpc-tU~HYAYSzLDl5XGGJU3YIg"
+            href="https://join.slack.com/t/firecracker-microvm/shared_invite/zt-3v81btcpe-usCf8Qk7k1gUlSAEKKdYMg"
             class="css-button3 sta-hoverOFF"
           >
             <i class="icon-slack" aria-hidden="true"></i>