Bump Rust to 1.75 (#3815)

**This PR also bumps wasmtime version from 0.38 to 2.0 as 0.38 does not
build with Rust 1.75**. I did not test the plugin runtime as (AFAIK) we
intend to deprecate it; also, wasmtime's most recent version is 16.0, so
it'd make sense to bump the version at some point anyways. I did not
bump the version to 16.0 straight away as that'd require code changes in
`plugin_runtime`.
Release Notes:

- N/A

crates/collab2/src/tests.rs

@@ -13,7 +13,6 @@ mod random_project_collaboration_tests;
 mod randomized_test_helpers;
 mod test_server;
 
-pub use crate as collab2;
 pub use randomized_test_helpers::{
     run_randomized_test, save_randomized_test_plan, RandomizedTest, TestError, UserTestPlan,
 };