feat: migrate environment variables to plugin properties

Standardize how environment variables are migrated to properties during
install triggers and migrate all remaining DOKKU_* config vars to their
appropriate plugin properties.

Adds a reusable MigrateConfigToProperties() function in the common
package with Transform callback and ListProperty support, plus a
migrate-config-to-property subcommand for the prop binary so shell
plugins can use the same code path.

Migrated variables and their new property owners:
- DOKKU_APP_PROXY_TYPE/DOKKU_PROXY_TYPE → proxy type
- DOKKU_DISABLE_PROXY → proxy disabled
- DOKKU_PROXY_PORT → ports proxy-port
- DOKKU_PROXY_SSL_PORT → ports proxy-ssl-port
- DOKKU_APP_RESTORE → ps restore
- DOKKU_SKIP_DEPLOY → ps skip-deploy
- DOKKU_START_CMD → ps start-cmd
- DOKKU_DOCKERFILE_START_CMD → ps dockerfile-start-cmd
- DOKKU_DISABLE_APP_AUTOCREATION → apps disable-autocreation
- DOKKU_APP_SHELL → scheduler shell
- DOKKU_SKIP_CLEANUP → builder skip-cleanup
- DOKKU_CHECKS_DISABLED → checks disabled
- DOKKU_CHECKS_SKIPPED → checks skipped
- DOKKU_CHECKS_WAIT → checks wait
- DOKKU_CHECKS_TIMEOUT → checks timeout
- DOKKU_CHECKS_ATTEMPTS → checks attempts
- DOKKU_DEFAULT_CHECKS_WAIT → checks default-wait
- DOKKU_SKIP_ALL_CHECKS → checks disabled (legacy)
- DOKKU_SKIP_DEFAULT_CHECKS → checks skipped (legacy)

Also refactors existing bespoke migration loops in scheduler, ports, ps,
builder, and nginx-vhosts plugins to use the standardized utility.

Removes DOKKU_PARALLEL_ARGUMENTS from documentation (unused in core).
Deprecates fn-migrate-config-to-property bash function.

closes #1558

plugins/proxy/subcommands.go

@@ -5,7 +5,6 @@ import (
 	"strings"
 
 	"github.com/dokku/dokku/plugins/common"
-	"github.com/dokku/dokku/plugins/config"
 )
 
 // CommandBuildConfig rebuilds config for a given app
@@ -99,12 +98,6 @@ func CommandSet(appName string, proxyType string) error {
 		return errors.New("Consider using ports:set command or specifying a valid proxy")
 	}
 
-	key := "DOKKU_APP_PROXY_TYPE"
-	if appName == "--global" {
-		key = "DOKKU_PROXY_TYPE"
-	}
-	entries := map[string]string{
-		key: proxyType,
-	}
-	return config.SetMany(appName, entries, false, false)
+	common.CommandPropertySet("proxy", appName, "type", proxyType, DefaultProperties, GlobalProperties)
+	return nil
 }