Move env_tags under env key

Instead of:

```
env:
  CLEAR_TAG: untagged
env_tags:
  tag1:
    CLEAR_TAG: tagged
```

We'll have:

```
env:
  clear:
    CLEAR_TAG: untagged
  tags:
    tag1:
      CLEAR_TAG: tagged
```

lib/kamal/configuration.rb

@@ -234,7 +234,7 @@ class Kamal::Configuration
   end
 
   def env_tags
-    raw_config.env_tags.collect { |name, config| Kamal::Configuration::Env::Tag.new(name, config: config) }
+    raw_config.env["tags"].collect { |name, config| Kamal::Configuration::Env::Tag.new(name, config: config) }
   end
 
   def env_tag(name)