Remove the misleading text in mailu.env that zstd and lz4 are supported for dovecot mail compression.

Zstd and lz4 are not supported. The reason is that the alpine project does not compile this
into the dovecot package.
Users who want this funcionality, can kindly request the alpine project to compile dovecot
with lz4&zstd support.

setup/flavors/compose/mailu.env

@@ -96,7 +96,7 @@ WELCOME_SUBJECT={{ welcome_subject or 'Welcome to your new email account' }}
 WELCOME_BODY={{ welcome_body or 'Welcome to your new email account, if you can read this, then it is configured properly!' }}
 
 # Maildir Compression
-# choose compression-method, default: none (value: gz, bz2, lz4, zstd)
+# choose compression-method, default: none (value: gz, bz2)
 COMPRESSION={{ compression }}
 # change compression-level, default: 6 (value: 1-9)
 COMPRESSION_LEVEL={{ compression_level }}