sanitize version_string we got from the wire (#8582)

`version_string` is meant to be displayed by UI, and comes from the
wire.

therefore, as a general precaution, ensure a string that is
regarded as a typical version string. all versions in scope are
currently v123.456.789-shortsuffix, where suffix is a-z and mostly
unused in production. that is the base. we can adapt if there is really
a need, but not for theoretical version strings. as we do not stop
processing, things are not bad even if we missed a valid usecase herr.

moreover, if `version_string` is empty, we skip the candidate - as we
cannot display something useful to the user. that little bit of care is
expected from relays :)

deltachat-jsonrpc/src/api/types/appversions.rs

@@ -9,7 +9,9 @@ pub struct JsonrpcAppSource {
     /// Always increasing version number.
     pub version_integer: u32,
 
-    /// Any version string.
+    /// Version string that should be shown to the user.
+    /// UI must not linkify the string
+    /// as it may be interpreted like a phone number or an IP address.
     pub version_string: String,
 
     /// Where to download that version.