update lint list + enable unnecessary_string_interpolations

[a14n]

Description

Update lint list and enable unnecessary_string_interpolations

Related Issues

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Tests

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[dnfield]

I feel a little bit squeamish about this without having NNBD, because string interpolation guards against null.

But LGTM.

[a14n]

because string interpolation guards against null

In #49352 I changed ~80 occurrences with only 1 case where it guarded against null. It's quite uncommon.