Make enable_starttls configurable by envvars (#20321)

ENABLE_STARTTLS is designed to replace ENABLE_STARTTLS_AUTO by accepting
three values: 'auto' (the default), 'always', and 'never'. If
ENABLE_STARTTLS isn't provided, we fall back to ENABLE_STARTTLS_AUTO. In
this way, this change should be fully backwards compatible.

Resolves #20311

chart/values.yaml

@@ -77,7 +77,7 @@ mastodon:
     ca_file: /etc/ssl/certs/ca-certificates.crt
     delivery_method: smtp
     domain:
-    enable_starttls_auto: true
+    enable_starttls: 'auto'
     from_address: notifications@example.com
     openssl_verify_mode: peer
     port: 587