CI: drop support for clang-format10 and later

The only acceptable versions of clang-format are 8 and 9 for now
(until another round of big-churn-from-formatting, at which point
we'll update the required version).

clang-format-9 says:

    SpacesInSquareBrackets (bool)
      If true, spaces will be inserted after [ and before ]. Lambdas
      or unspecified size array declarations will not be affected.

clang-format-10 changes part of that to:

      Lambdas without arguments or unspecified size array
      declarations will not be affected.

This means that 9 will only allow `[name]` for captures, and 10
will only allow `[ name ]` for captures, so they ping-pong all
the lambda's in the codebase back and forth. Just don't.

ci/calamaresstyle

@@ -55,14 +55,8 @@ case "$format_version" in
 	[89] )
 		:
 		;;
-	10 )
-		extra_settings="10"
-		;;
-	11 )
-		extra_settings="10 11"
-		;;
 	* )
-		echo "! Clang-format version '$format_version' unknown."
+		echo "! Clang-format version '$format_version' unsupported."
 		exit 1
 		;;
 esac