Upgrade to Zephyr 3.1

We decided to go with 3.1 for the following reasons:

2.7 LTS will require lots of backports to get LVGL working
There is no product currently using 2.7
3.2 could become the next Zephyr LTS so we'd have a smaller delta to upgrade to it when needed.
going to 3.1 allows us to gain from the additional platform support (e.g. esp32 and risc-v) and reduces our maintenance workload

Edited Jul 22, 2022 by Amit Kucheria

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