Merge bitcoindevkit/bdk#1850: ci: pin `flate2` for msrv

2cf9991ea84ecec68b8c7cd39773e61e9e5ca36a ci: pin `flate2` for msrv (Wei Chen)

Pull request description:

  ### Description
  `flate2` 1.1.0 raised msrv to 1.67. Previous version has been pinned.

  ### Changelog notice

  * Pinned flate2 dependency to 1.0.35.

  ### Checklists

  #### 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:
  ValuedMammal:
    ACK 2cf9991ea84ecec68b8c7cd39773e61e9e5ca36a

Tree-SHA512: ef77117a6037562d1bf2fbc1d4fcc5b49dd91c965775b008d25379a258e17c93edeb664c212877189568493b3f00b8663df0a5bc374802c487961d36bffa9dfe