settings, upgrade: Add dovecot_storage_version

data/settings.js

@@ -3533,6 +3533,17 @@ configuration changes, the old configuration will either keep working or there
 will be a clear failure at startup.`
 	},
 
+	dovecot_storage_version: {
+		values: setting_types.STRING,
+		text: `
+Dovecot storage file format version. It specifies the oldest Dovecot version
+that must be able to read files written by this Dovecot instance. The intention
+is that when upgrading Dovecot cluster, this setting is first kept as the old
+Dovecot version. Once the cluster is fully upgraded to a new version and
+there is no intention to rollback to the old version anymore, this version
+number can be increased.`
+	},
+
 	dsync_alt_char: {
 		default: '_',
 		tags: [ 'dsync' ],

docs/core/man/include/global-options.inc

@@ -6,7 +6,9 @@ Global [[man,doveadm]] *options*:
 :   Enables *verbosity* and debug messages.
 
 **-O**
-:   Do not read any config file, just use defaults.
+:   Do not read any config file, just use defaults. The
+    [[setting,dovecot_storage_version]] defaults to the latest version, but
+    can be overridden with **-o**.
 
 **-k**
 :   Preserve entire environment for doveadm, not just

docs/installation/upgrade/include/2.3-to-2.4.inc

@@ -6,6 +6,9 @@ The first setting in `dovecot.conf` must now be
 [[setting,dovecot_config_version]]. This helps to avoid unexpected
 configuration changes in the future.
 
+Another new required setting is [[setting,dovecot_storage_version]]. This helps
+to avoid unexpected storage file format incompatibilities.
+
 passdb and userdb sections now require a name, i.e.: