website/docs: update notes on SECRET_KEY (#9091)

* website/docs:  update notes on SECRET_KEY

Signed-off-by: pidi3000 <76975863+pidi3000@users.noreply.github.com>

* Update website/docs/installation/configuration.mdx

Co-authored-by: Tana M Berry <tanamarieberry@yahoo.com>
Signed-off-by: pidi3000 <76975863+pidi3000@users.noreply.github.com>

* reword

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

---------

Signed-off-by: pidi3000 <76975863+pidi3000@users.noreply.github.com>
Signed-off-by: Jens Langhammer <jens@goauthentik.io>
Co-authored-by: Tana M Berry <tanamarieberry@yahoo.com>
Co-authored-by: Jens Langhammer <jens@goauthentik.io>

website/docs/installation/configuration.mdx

@@ -150,7 +150,11 @@ These settings affect where media files are stored. Those files include applicat
 
 ### `AUTHENTIK_SECRET_KEY`
 
-Secret key used for cookie signing and unique user IDs. Do not change this after the first install.
+Secret key used for cookie signing. Changing this will invalidate active sessions.
+
+:::caution
+Prior to 2023.6.0 the secret key was also used for unique user IDs. When running a pre-2023.6.0 version of authentik the key should _not_ be changed after the first install.
+:::
 
 ### `AUTHENTIK_LOG_LEVEL`