disable blank-canvas & kansas-comet (#7136)

also disable diagrams-canvas via its dependency on 'blank-canvas'

build-constraints.yaml

@@ -1207,7 +1207,7 @@ packages:
         - diagrams
         - diagrams-builder
         - diagrams-cairo
-        - diagrams-canvas
+        - diagrams-canvas < 0 # by way of 'blank-canvas': https://github.com/commercialhaskell/stackage/issues/7136
         - diagrams-contrib
         - diagrams-core
         - diagrams-gtk
@@ -5321,7 +5321,7 @@ packages:
         - bindings-DSL
         - bindings-uname
         - bitarray
-        - blank-canvas
+        - blank-canvas < 0 # https://github.com/commercialhaskell/stackage/issues/7136
         - blaze-svg
         - blaze-textual
         - bloomfilter
@@ -5527,7 +5527,7 @@ packages:
         - json
         - json-alt
         - jsonrpc-tinyclient
-        - kansas-comet
+        - kansas-comet < 0 # https://github.com/commercialhaskell/stackage/issues/7136
         - kleene
         - knob
         - language-haskell-extract