Replace Boost_VERSION with Boost_VERSION_STRING in STIRConfig.cmake

[robbietuk]

Changes in this pull request

This pull request includes a modification to the src/cmake/STIRConfig.cmake.in file to correct the version specification for the Boost library.

Testing performed

Fixes my issue in #1541

Related issues

Fixes #1541

Checklist before requesting a review

• I have performed a self-review of my code
• [] I have added docstrings/doxygen in line with the guidance in the developer guide
• [] I have implemented unit tests that cover any new or modified functionality (if applicable)
• The code builds and runs on my machine
• [] documentation/release_XXX.md has been updated with any functionality change (if applicable)