[MMTags] Fix the definition of the CHANNEL_MINS and CHANNEL_MAXES tags.

The previous values are not used anywhere else in the codebase, making them
useless for controlling how the images are displayed. They've been replaced
by "ChContrastMin" and "ChContrastMax".

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14838 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd