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/common/functions

@@ -632,8 +632,8 @@ release_and_deploy() {
 
   if verify_image "$IMAGE"; then
     IMAGE_SOURCE_TYPE="$(get_image_builder_type "$IMAGE" "$APP")"
-    local DOKKU_APP_SKIP_DEPLOY="$(config_get "$APP" DOKKU_SKIP_DEPLOY || true)"
-    local DOKKU_GLOBAL_SKIP_DEPLOY="$(config_get --global DOKKU_SKIP_DEPLOY || true)"
+    local DOKKU_APP_SKIP_DEPLOY="$(fn-plugin-property-get-default "ps" "$APP" "skip-deploy" "")"
+    local DOKKU_GLOBAL_SKIP_DEPLOY="$(fn-plugin-property-get-default "ps" "--global" "skip-deploy" "")"
 
     local DOKKU_SKIP_DEPLOY=${DOKKU_APP_SKIP_DEPLOY:="$DOKKU_GLOBAL_SKIP_DEPLOY"}
 
@@ -917,7 +917,7 @@ suppress_output() {
 }
 
 fn-migrate-config-to-property() {
-  declare desc="migrates deprecated config variables to property counterpart"
+  declare desc="[deprecated] migrates deprecated config variables to property counterpart. Use 'prop migrate-config-to-property' instead."
   declare PLUGIN="$1" KEY="$2" CONFIG_VAR="$3" GLOBAL_CONFIG_VAR="$4"
 
   # todo: refactor to remove config_unset usage