Merge bitcoindevkit/bdk#1883: ci: pin `once_cell` to `1.20.3` for MSRV

Merge bitcoindevkit/bdk#1883: ci: pin `once_cell` to `1.20.3` for MSRV

2db868fa47c87163a55352387ff99466ea22813a ci: pin `once_cell` to `1.20.3` for MSRV (Wei Chen)

Pull request description:

  ### Description

  Pin `once_cell` to `1.20.3` for MSRV.

  ### 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 2db868fa47c87163a55352387ff99466ea22813a

Tree-SHA512: d1738efd3f126a896157dd06d1366dcd9b90221184e422418e510c98c770b1fc7c479af0ec8c3efa5992775d0296e7ce8b5c546a8f44a800d706202738fdee59