Correct our proptest requirement (#3256)

## Motivation

Our code currently doesn't build with `proptest` <1.6, as we rely on
`std::ops::RangeFrom<u128>: proptest::strategy::Strategy`. This is
reflected in our lock file but not our manifest, possibly breaking
builds that depend on our crates.

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## Proposal

Update the manifest.

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## Test Plan

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## Release Plan

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- Nothing to do / These changes follow the usual release cycle.

## Links

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- [reviewer
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Cargo.toml

@@ -115,7 +115,7 @@ prettyplease = "0.2.16"
 prometheus = "0.13.3"
 proc-macro-error = "1.0.4"
 proc-macro2 = "1.0"
-proptest = { version = "1.4.0", default-features = false, features = ["alloc"] }
+proptest = { version = "1.6.0", default-features = false, features = ["alloc"] }
 prost = "0.13.2"
 quote = "1.0"
 rand = { version = "0.8.5", default-features = false }