Merge bitcoindevkit/bdk#1803: ci: add secondary MSRV job of 1.75.0

c8b55da39913ddaf11d26f09d92034aef4f550e0 chore: fix punctuation and use `rustup override set` instead (Leonardo L.)
78bbfbe7210959b2c174c6c7e418bc975055a6ca ci: add `pin-msrv.sh` (valued mammal)
f7c9bc5166a508743ad11f116131a6f94d3e8e9e docs: update `README.md` with MSRV policy (valued mammal)
12eabd1ecd30bfed1d1329966029ea8c4cc34ac1 chore(cleanup): remove rustls from pinned deps (Leonardo Lima)
e3faeb63a07e4b8174de7cb521b54fae6f52779f ci: add secondary MSRV job of 1.75.0 (Leonardo Lima)

Pull request description:

  partially fixes #1750

  <!-- You can erase any parts of this template not applicable to your Pull Request. -->

  ### Description

  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->

  It adds a secondary MSRV version of `1.75.0` for the `bdk_electrum` crate, excluding it from the `1.63.0` MSRV dependency pinning and CI step.

  As it's the new CI will run without `bdk_electrum` for `1.63.0` MSRV step, will run another step for `1.75.0` MSRV and the latest stable.

  Currently,, the only affected crate is `electrum-client`, therefore `bdk-electrum`, as it relies directly on `rustls` which migrated to `1.71.0` MSRV. The `esplora-client` relies on `rustls` as a dependency of `minreq` or `reqwest`, but those crates didn't upgraded it's `rustls` versions yet.

  In a further PR it should also bump the `electrum-client` crate to it's latest version, which relies on `1.75.0` MSRV, see https://github.com/bitcoindevkit/rust-electrum-client/pull/159.

  ### Notes to the reviewers

  <!-- In this section you can include notes directed to the reviewers, like explaining why some parts
  of the PR were done in a specific way -->

  It's open for discussion if this approach is the expected one, or another one would be better.

  ### Changelog notice

  <!-- Notice the release manager should include in the release tag message changelog -->
  <!-- See https://keepachangelog.com/en/1.0.0/ for examples -->

  - Add a secondary MSRV of `1.75.0` for `bdk_electrum`.

  ### 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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

ACKs for top commit:
  notmandatory:
    ACK c8b55da39913ddaf11d26f09d92034aef4f550e0

Tree-SHA512: 186d82d6d295c688a608e1dd457e2718e0ae3ee92b7d1a0aad094711c3baeb70dccd581e3067e6305c1fc702c78f1704399f7472a5ae62824041ac68c384b770