[Rust] Defined MSRV at 1.88

[cartertinney]

• Updated all crates to indicate MSRV (Minimum Supported Rust Version) is 1.88.
• Note that this is just a formalization of what we've been using by convention up until now - it's possible we could loosen this restriction further in the future, but would require investigation.
• Can be set in the workspace for most crates, but the connector scaffolding needs to be independently defined due to not being part of the workspace (and being a scaffold for customer applications that will not use our workspace)
• CI that runs both MSRV and latest stable will be a follow up

[Copilot]

Pull request overview

This PR formalizes the Rust Minimum Supported Rust Version (MSRV) across the Rust workspace as Rust 1.88, aligning crate metadata and local developer tooling with the version already used by convention.

Changes:

• Set workspace-wide MSRV via rust/Cargo.toml and opt member crates into inheriting it via rust-version.workspace = true.
• Pin the Rust toolchain used for local development to 1.88 via rust/rust-toolchain.toml.
• Define MSRV explicitly for the excluded sample_connector_scaffolding crate since it is not part of the workspace.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rust/Cargo.toml	Defines workspace MSRV ([workspace.package] rust-version = "1.88").
rust/rust-toolchain.toml	Pins local dev toolchain channel to 1.88.
rust/azure_iot_operations_mqtt/Cargo.toml	Inherits MSRV from workspace.
rust/azure_iot_operations_protocol/Cargo.toml	Inherits MSRV from workspace.
rust/azure_iot_operations_services/Cargo.toml	Inherits MSRV from workspace.
rust/azure_iot_operations_connector/Cargo.toml	Inherits MSRV from workspace.
rust/sample_applications/counter/envoy/Cargo.toml	Inherits MSRV from workspace.
rust/sample_applications/counter/counter_server/Cargo.toml	Inherits MSRV from workspace.
rust/sample_applications/counter/counter_client/Cargo.toml	Inherits MSRV from workspace.
rust/sample_applications/event_driven_app/input_client/Cargo.toml	Inherits MSRV from workspace.
rust/sample_applications/event_driven_app/output_client/Cargo.toml	Inherits MSRV from workspace.
rust/sample_applications/sample_connector_scaffolding/Cargo.toml	Sets explicit MSRV (rust-version = "1.88") since it’s excluded from the workspace.