Update Readme.md

[smartm0use]

Fixed Rust version requirement

[RCasatta]

Need to investigate, CI is apparently testing for 1.65

[RCasatta]

from the docs this PR is right https://doc.rust-lang.org/stable/std/collections/struct.BTreeSet.html#method.first

[RCasatta]

Concept ACK (but I don't know exactly why CI is not catching this)
Please update also the github actions file to use 1.66.0

[RCasatta]

Could you please:

1. Squash the two commits 7d52b18 and 7d52b18 you usually don't want different commits of the same PR to touch the same lines
2. in another or same commit, update here

   fbbe/.github/workflows/test.yml

   Line 41 in 55a80c6

   toolchain: ["1.65.0", "stable", "nightly"]

   from 1.65 to 1.66

Thanks, sorry for being pedantic

[RCasatta]

utACK 03e3b19

Thanks!