Bump MSRV to 1.54.0

[rina23q]

MSRV to 1.54.0

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Further comments

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[ChrisGreenaway]

Can you say why this is necessary?

[rina23q]

Yes, tarpaulin gives an error to bitvec 0.19.5 if the toolchain version is 1.52 or higher. People reported this in xd009642/tarpaulin#756, and the solution is to add the argument.