doc: add C++17 release note for 0.21.0

Mention that the codebase is now compatible with C++17, and that the
intention is to require C++17 starting with 0.22.0.

doc/release-notes.md

@@ -45,6 +45,11 @@ wallet versions of Bitcoin Core are generally supported.
 Compatibility
 ==============
 
+During this release cycle, work has been done to ensure that the codebase is fully
+compatible with C++17. The intention is to begin using C++17 features starting
+with the 0.22.0 release. This means that a compiler that supports C++17 will be
+required to compile 0.22.0.
+
 Bitcoin Core is supported and extensively tested on operating systems
 using the Linux kernel, macOS 10.12+, and Windows 7 and newer.  Bitcoin
 Core should also work on most other Unix-like systems but is not as