net: don't accept non-left-contiguous netmasks

A netmask that contains 1-bits after 0-bits (the 1-bits are not
contiguous on the left side) is invalid [1] [2].

The code before this PR used to parse and accept such
non-left-contiguous netmasks. However, a coming change that will alter
`CNetAddr::ip` to have flexible size would make juggling with such
netmasks more difficult, thus drop support for those.

[1] https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing#Subnet_masks
[2] https://tools.ietf.org/html/rfc4632#section-5.1

doc/release-notes.md

@@ -135,6 +135,10 @@ Updated settings
   in future releases. Refer to the help of the affected settings `-whitebind`
   and `-whitelist` for more details. (#19191)
 
+- Netmasks that contain 1-bits after 0-bits (the 1-bits are not contiguous on
+  the left side, e.g. 255.0.255.255) are no longer accepted. They are invalid
+  according to RFC 4632.
+
 Changes to Wallet or GUI related settings can be found in the GUI or Wallet  section below.
 
 Tools and Utilities