Bump submodule containers to 0.6.7

It's critical to backport to ghc-9.6 since this release provides instance Foldable1 Data.Tree, otherwise compatibility packages would be in trouble.

This release also brings nice performance improvements:

Metrics: compile_time/bytes allocated
                          geo. mean                                          -1.0%     
                          minimum                                            -8.5%     
                          maximum                                            +0.2%