KNOWN_BUGS: cmake: libspsl is not supported

Closes #6214

docs/KNOWN_BUGS

@@ -131,6 +131,7 @@ problems may have been fixed or changed somewhat since this was written!
  15.8 libcurl.pc uses absolute library paths
  15.9 doesn't fail if zlib support is requested but not found
  15.10 cert paths autodetected when cross-compiling
+ 15.11 libspsl is not supported
 
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@@ -896,3 +897,7 @@ problems may have been fixed or changed somewhat since this was written!
  cross-compiling. The cmake build keeps doing the detection.
 
  See https://github.com/curl/curl/issues/6178
+
+15.11 libspsl is not supported
+
+ See https://github.com/curl/curl/issues/6214