deps: update to ExecHandlers for mvdan.cc/sh

[dtrudg]

Type of issue

tech debt

Description of issue

In mvdan.cc/sh v3.7.0, the setting of an interpreter ExecHandler was deprecated in favour of one or more ExecHandlers.

I have failed to make a single entry ExecHandlers work in the same way as ExecHandler, so for now the deprecation lint failure has been turned off.

We need to switch to the new ExecHandlers to allow migration to a future v4.

[dtrudg]

Had another go at this today, and still can't make ExecHandlers work with only a single entry... no default / fallback.... so going to not let this block a 4.2 RC.