Only run clippy for the stable rust version

[notmandatory]

Description

It was decided during the team call today (2021-12-14) to only run clippy for the stable rust version.

Notes to the reviewers

This is required to fix the below build issues when running clippy on rust version 1.46.0.

cargo clippy --all-targets --features async-interface --no-default-features -- -D warnings

...

Checking bitcoincore-rpc v0.14.0
error: unknown clippy lint: clippy::no_effect_underscore_binding
  --> src/blockchain/mod.rs:88:1
   |
88 | #[maybe_async]
   | ^^^^^^^^^^^^^^
   |
   = note: `-D clippy::unknown-clippy-lints` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unknown_clippy_lints
   = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: unknown clippy lint: clippy::no_effect_underscore_binding
   --> src/blockchain/mod.rs:220:1
    |
220 | #[maybe_async]
    | ^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unknown_clippy_lints
    = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing