Declare Rust MSRV and warn against distro-packaged toolchains

[AnonRish]

Summary

• Declare rust-version = "1.85" under [workspace.package], matching
  the edition 2024 requirement already declared there
• State the 1.85+ minimum explicitly in the prerequisites section, next
  to the existing protoc-version rationale
• Warn against distro-packaged Rust toolchains, which are commonly older
  than edition 2024 requires

Problem

The workspace sets edition = "2024" (requires rustc >=1.85) but no
Cargo.toml declares rust-version, and no doc states a minimum. Installing
via a Linux distribution's package manager -- a natural reading of "install
a Rust toolchain," and what many CI base images ship by default -- commonly
gives a too-old toolchain. On Ubuntu 24.04 (rustc 1.75.0) this fails with:

feature `edition2024` is required
The package requires the Cargo feature called `edition2024`, but that
feature is not stabilized in this version of Cargo (1.75.0).

which reads like an unstable/nightly-feature problem rather than a version
mismatch.

Fix

• Add rust-version = "1.85", standard MSRV metadata that crates.io and
  MSRV-aware tooling read.
• State the requirement in prose, next to the protoc callout that already
  explains why an old version fails the same way.

Verification

• Reproduced the failure above on a clean Ubuntu 24.04 container with
  apt install rustc cargo.
• Confirmed no Cargo.toml in the repo declared rust-version and no doc
  mentioned a minimum version, via grep across **/*.md and
  **/Cargo.toml.

A toolchain old enough to reject edition2024 outright (1.75.0, tested
above) fails at manifest-parse time, before rust-version is checked, so
this doesn't change that specific error message -- it's correct, standard
metadata regardless, and should produce a clean version error on
toolchains between "supports rust-version" and "supports edition2024." The
doc change is the part verified to fix the reported symptom directly.