ci: minimal version check (v0.2.x)

[Darksonn]

This checks that crates compile with the minimum allowed version of each dependency.

Backport of #3131

[taiki-e]

error[E0599]: no method named drain found for struct slab::Slab<std::thread::JoinHandle<()>> in the current scope
--> tokio/src/runtime/blocking/pool.rs:139:35
|
139 | for handle in workers.drain() {
| ^^^^^ method not found in slab::Slab<std::thread::JoinHandle<()>>

It seems slab's requirement needs to be raised to 0.4.2.
tokio-rs/slab@9c114f0

[taiki-e]

error: cannot find macro log in this scope
--> tokio-util/src/codec/framed_impl.rs:215:9
|
215 | trace!("framed transport flushed");
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It seems log's requirement needs to be raised to 0.4.6 (or 0.4.4).
rust-lang/log@11dfde1
rust-lang/log@2f52d6c