Pin the version of the base64ct indirect dependency (#3420)

## Motivation

Version 1.7.0 breaks semver compatibility by requiring the usage of a
new Rust edition. More information is available in
RustCrypto/formats#1684.

## Proposal

We should pin it to the last version that we still support at least
until we manage to update to the new Rust edition.

## Test Plan

CI should catch any regressions.

## Release Plan

We may have to make a new SDK release with a new patch version.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

linera-sdk/Cargo.toml

@@ -18,6 +18,10 @@ development = ["linera-sdk"]
 features = ["test", "wasmer"]
 targets = ["wasm32-unknown-unknown", "x86_64-unknown-linux-gnu"]
 
+[package.metadata.cargo-machete]
+# TODO(#3421): Remove the pinned version once the `linera-*` crates move to Rust Edition 2024
+ignored = ["base64ct"]
+
 [features]
 ethereum = ["async-trait", "linera-ethereum"]
 unstable-oracles = ["linera-core/unstable-oracles", "linera-execution/unstable-oracles"]
@@ -56,6 +60,9 @@ serde_json.workspace = true
 thiserror.workspace = true
 wit-bindgen.workspace = true
 
+# TODO(#3421): Remove the pinned version once the `linera-*` crates move to Rust Edition 2024
+base64ct = "=1.6.0"
+
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 async-graphql.workspace = true
 anyhow.workspace = true