chore: Add news fragment

Some people might be interested I guess

changelog.d/2189.feature.md

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+Removed the ability to run a server with "challengeless registration" (i.e. the big scary long config option, with no captcha, email, or registration token requirements), without manually compiling
+the server with non-default arguments. Contributed by @nex.
+
+(This is part of a wider effort to reduce abuse in the Matrix network, of which servers that are easily automated cause a disproportionate amount of)