Merge bitcoindevkit/bdk#1564: ci: add cron-update-rust.yml

b92f8c9ac1e000ccced89dc597a8ff278842d5ed ci: add cron-update-rust.yml (valued mammal)

Pull request description:

  This adds a cronjob to watch for updates to rust stable once a month. If the latest version changed, then a pull request is opened to see if clippy passes CI using the new toolchain.

  ### Notes to the reviewers

  Let me know what you think @notmandatory

  fixes #1431

  ### Changelog notice

  ### Checklists

  #### TODO

  * [ ] Setup access token for [create-pull-request](https://github.com/marketplace/actions/create-pull-request)

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  oleonardolima:
    tACK b92f8c9ac1e000ccced89dc597a8ff278842d5ed

Tree-SHA512: e251fb8d0456c30a244f4611131baea1afc4f8323a3bdc8d9536d276a8b4bb2c0269beba6b04b72ab2ea96897e421a16c3e8e6065a755d62ff98bfe13d0b02bd