`modify_network`: add `--ex-date12` option

[yunjunz]

Description of proposed changes

This PR adds the capability to exclude interferometric pairs given the list of date12, besides given the ifgram index. The benefit is that it won't change after updating interferogram stack.

• (cli.)modify_network.py: add --exclude-date12 option to exclude pairs given date1_date2

• defaults.smallbaselineApp(_auto).cfg: add mintpy.network.excludeDate12 option with auto value of no

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