Merge bitcoindevkit/bdk#1886: ci: pin `minreq` to `2.13.2`, `base64ct` to `1.6.0...

a186f00d9846118a3034fda2a4eb4393548ceb90 ci: pin `minreq` to `2.13.2` (Luis Schwab)
1a3b420bcbbd346555a38a6b134e13df3e22593a ci: pin `base64ct` to `1.6.0` (Luis Schwab)

Pull request description:

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

  ### Description

  - Pin `minreq` to `2.13.2` because `2.13.3` uses the unstable `std::sync::LazyLock` (https://github.com/bitcoindevkit/bdk/pull/1886#issuecomment-2714736611).
  - Pin `base64ct` to `1.6.0` for MSRV.

  Required for #1885 (`ubuntu-24.04` on all CI workflows).

  ### 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:
  LagginTimes:
    ACK a186f00d9846118a3034fda2a4eb4393548ceb90
  ValuedMammal:
    ACK a186f00d9846118a3034fda2a4eb4393548ceb90

Tree-SHA512: cfc78846c3225caa28bcd985bdec670ffccd564b6b8c4b9e9d6c8fdb3bf923005704118e994f1e211f8a52b7ddb34d9fe9b30d89a1d5254eca146f1720466e49