Describe the enhancement

As noted by @monaka in #1012, env_logger versions below 0.10 uses atty (which is unmaintained). BDK should allow projects with higher MSRVs to use env_logger.

@apoelstra suggested in #1012 (comment):

If the dep can really be bumped with no changes to the codebase, it may be possible to provide a ranged version that covers multiple major versions.

If this is even possible, we need to update CI to test for every version of env_logger in our specified range using cargo update --precise.