Merge #338: ci(msrv): pin syn crate to 2.0.106

f146b0a ci(msrv): pin syn crate to 2.0.106 (Steve Myers)

Pull request description:

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

  ### Description

  Pinning to `syn` 2.0.106 which has MSRV 1.61.0.

  ### Notes to the reviewers

  The latest `syn` 2.0.107 and 2.0.108 have MSRV 1.68.0, which breaks our MSRV tests.

  ### 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 `just p` before pushing

ACKs for top commit:
  ValuedMammal:
    ACK f146b0a

Tree-SHA512: d3867f0d623a66d922cc2a35bb5cd6e6a5f2ce5b5fa704e150cc4edbfe018f4796a9a571c4806eadd5ff78265b8315dbe17ba8c47b05fe92d487221923086df6

Author: ValuedMammal

ci/pin-msrv.sh

@@ -11,3 +11,4 @@ set -euo pipefail
 # rustup override set 1.63.0
 
 cargo update -p once_cell --precise "1.20.3"
+cargo update -p syn --precise "2.0.106"