chore: unpin macOS Boost from 1.85 to latest

The boost@1.85 pin was re-introduced when merging main (which
contains PR #505's pinned version). This commit re-applies the
unpin so macOS CI builds against the latest Homebrew Boost (1.90),
exercising the new BOOST_VERSION >= 109000 code path in foxy.

Co-Authored-By: rlamb@launchdarkly.com <kingdewman@gmail.com>

Author: devin-ai-integration[bot]

.github/actions/install-boost/action.yml

@@ -43,8 +43,8 @@ runs:
       if: runner.os == 'macOS'
       shell: bash
       run: |
-        brew install boost@1.85
-        echo "BOOST_ROOT=$(brew --prefix boost@1.85)" >> $GITHUB_OUTPUT
+        brew install boost
+        echo "BOOST_ROOT=$(brew --prefix boost)" >> $GITHUB_OUTPUT
 
     - name: Determine root
       id: determine-root