Fix colourized output to file

This change should be backwards compatible, but it would be better
to make the console colour API take an argument in the long term.

Fixes #1502

docs/deprecations.md

@@ -83,6 +83,11 @@ be changed so that hidden tests are included in a run only if they
 positively match a testspec.
 
 
+### Console Colour API
+
+The API for Catch2's console colour will be changed to take an extra
+argument, the stream to which the colour code should be applied.
+
 ---
 
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