chore: bump ci rust version (#1543)
This PR bumps the rust toolchain in CI to resolve the CI build issue
```bash
Downloaded crossbeam-utils v0.8.19
Downloaded crc32fast v1.3.2
error: failed to compile `text-generation-router v1.4.0 (/home/runner/work/text-generation-inference/text-generation-inference/router)`, intermediate artifacts can be found at `/home/runner/work/text-generation-inference/text-generation-inference/target`
Caused by:
package `clap_lex v0.7.0` cannot be built because it requires rustc 1.74 or newer, while the currently active rustc version is 1.71.0
Either upgrade to rustc 1.74 or newer, or use
cargo update -p clap_lex@0.7.0 --precise ver
where `ver` is the latest version of `clap_lex` supporting rustc 1.71.0
make: *** [Makefile:12: install-router] Error 101
```
