Merge bitcoindevkit/bdk#2368: ci: pin proptest to 1.11.0 for MSRV

Merge bitcoindevkit/bdk#2368: ci: pin proptest to 1.11.0 for MSRV

1a457c8253d588d927e1be866654cbebaccf15ed ci: pin proptest to 1.11.0 for MSRV (志宇)

Pull request description:

  ### Description

  The MSRV (1.85.0) CI job is failing on `master` and on every open PR. `proptest` 1.12.0 declares `rust-version = "1.88"`, and since there is no committed lockfile, CI resolves to it:

  ```
  error: rustc 1.85.0 is not supported by the following package:
    proptest@1.12.0 requires rustc 1.88
  ```

  `proptest` is a dev-dependency of `bdk_core` and `bdk_chain` (`proptest = "1.2.0"`). This PR pins it to 1.11.0 in `ci/pin-msrv.sh`, the latest release that declares `rust-version = "1.85"`.

  Failing run on `master`: https://github.com/bitcoindevkit/bdk/actions/runs/38205608516/job/114668304248

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  🤖 Generated with [Claude Code](https://claude.com/claude-code)

ACKs for top commit:
  notmandatory:
    ACK 1a457c8253d588d927e1be866654cbebaccf15ed
  noahjoeris:
    tACK 1a457c8253d588d927e1be866654cbebaccf15ed

Tree-SHA512: a8361dbc5604935a409a280c324e9fae495d30d6a8699a83932d48b938ae5eb3f3b1bff2d2945b145cc91aeb9062bc49230fc8c8fa652c30e513686a8c0ce616