Enforce MSRV via Cargo and use stable toolchain. Fixes #14.

[tripathiakshit]

The install script no longer forces/pins a specific Rust toolchain. Instead, it checks for a minimum Rust version and proceeds if the system rustc is newer than the MSRV.

Changes:

• install.sh: add robust version comparison (version_ge) and require only rustc >= 1.88.0; improve rustc presence check and error messages.
• service/Cargo.toml: set rust-version = 1.88.0 to declare MSRV.
• rust-toolchain.toml: switch channel from pinned "1.88" to "stable" so newer rustc toolchains are respected.

Why:

• Fixes behavior where ./scripts/install.sh would attempt to install/downgrade to Rust 1.88.0 even when a newer rustc (e.g., 1.89.0) is already installed.
• Aligns with expected behavior: proceed with installation if the installed Rust version meets or exceeds the MSRV.

Result:

• Systems with rustc >= 1.88.0 (e.g., 1.89.0) will pass the check and install without rustup toolchain changes.

[can1357]

Tyvm!

[can1357]

Seems like clippy is failing though so I'll have to check at home.