Fix pickle_key configuration value for mautrix-slack for compatibility with the old bridge

roles/custom/matrix-bridge-mautrix-slack/defaults/main.yml

@@ -148,6 +148,9 @@ matrix_mautrix_slack_registration: "{{ matrix_mautrix_slack_registration_yaml |
 matrix_mautrix_slack_bridge_encryption_allow: "{{ matrix_bridges_encryption_enabled }}"
 matrix_mautrix_slack_bridge_encryption_default: "{{ matrix_bridges_encryption_default }}"
 matrix_mautrix_slack_bridge_encryption_key_sharing_allow: "{{ matrix_mautrix_slack_bridge_encryption_allow }}"
+# This pickle key value is backward-compatible with the old bridge.
+# See: https://github.com/mautrix/slack/releases/tag/v0.1.0
+matrix_mautrix_slack_bridge_encryption_pickle_key: maunium.net/go/mautrix-whatsapp
 
 matrix_mautrix_slack_provisioning_shared_secret: ''
 matrix_mautrix_slack_public_media_signing_key: ''

roles/custom/matrix-bridge-mautrix-slack/templates/config.yaml.j2

@@ -376,7 +376,7 @@ encryption:
     allow_key_sharing: {{ matrix_mautrix_slack_bridge_encryption_key_sharing_allow | to_json }}
     # Pickle key for encrypting encryption keys in the bridge database.
     # If set to generate, a random key will be generated.
-    pickle_key: generate
+    pickle_key: {{ matrix_mautrix_slack_bridge_encryption_pickle_key | to_json }}
     # Options for deleting megolm sessions from the bridge.
     delete_keys:
         # Beeper-specific: delete outbound sessions when hungryserv confirms