Update debian install docs for new key and repo (#5074)

INSTALL.md

@@ -257,18 +257,38 @@ https://github.com/spantaleev/matrix-docker-ansible-deploy
 #### Matrix.org packages
 
 Matrix.org provides Debian/Ubuntu packages of the latest stable version of
-Synapse via https://matrix.org/packages/debian/. To use them:
+Synapse via https://packages.matrix.org/debian/. To use them:
 
+For Debian 9 (Stretch), Ubuntu 16.04 (Xenial), and later:
+
+```
+sudo apt install -y lsb-release wget apt-transport-https
+sudo wget -O /usr/share/keyrings/matrix-org-archive-keyring.gpg https://packages.matrix.org/debian/matrix-org-archive-keyring.gpg
+echo "deb [signed-by=/usr/share/keyrings/matrix-org-archive-keyring.gpg] https://packages.matrix.org/debian/ $(lsb_release -cs) main" |
+    sudo tee /etc/apt/sources.list.d/matrix-org.list
+sudo apt update
+sudo apt install matrix-synapse-py3
 ```
-sudo apt install -y lsb-release curl apt-transport-https
-echo "deb https://matrix.org/packages/debian `lsb_release -cs` main" |
+
+For Debian 8 (Jessie):
+
+```
+sudo apt install -y lsb-release wget apt-transport-https
+sudo wget -O /etc/apt/trusted.gpg.d/matrix-org-archive-keyring.gpg https://packages.matrix.org/debian/matrix-org-archive-keyring.gpg
+echo "deb [signed-by=5586CCC0CBBBEFC7A25811ADF473DD4473365DE1] https://packages.matrix.org/debian/ $(lsb_release -cs) main" |
     sudo tee /etc/apt/sources.list.d/matrix-org.list
-curl "https://matrix.org/packages/debian/repo-key.asc" |
-    sudo apt-key add -
 sudo apt update
 sudo apt install matrix-synapse-py3
 ```
 
+**Note**: if you followed a previous version of these instructions which
+recommended using `apt-key add` to add an old key from
+`https://matrix.org/packages/debian/`, you should note that this key has been
+revoked. You should remove the old key with `sudo apt-key remove
+C35EB17E1EAE708E6603A9B3AD0592FE47F0DF61`, and follow the above instructions to
+update your configuration.
+
+
 #### Downstream Debian/Ubuntu packages
 
 For `buster` and `sid`, Synapse is available in the Debian repositories and