build(rust): declare rust-version 1.88; rustup setup for the server

scripts/install_collector.sh

@@ -12,7 +12,8 @@
 # linux-x86_64" CI artifact) or from rust/target/release/ping-collector, built
 # beforehand as a normal user:
 #   cargo build --release --locked --manifest-path rust/Cargo.toml
-# (needs rustup, pkg-config and libssl-dev).
+# (needs Rust >= 1.88 from rustup -- distro cargo is too old -- plus pkg-config
+# and libssl-dev; see rust/README.md).
 set -euo pipefail
 
 APP_DIR="${APP_DIR:-/opt/thailand-water-monitor}"