Improve CMake and Actions settings

[doug-walker]

Several minor build fixes:

• Fix a CMake error in how OCIO_NAMESPACE was being used. This was discussed at the TSC meeting and the preference was for this to continue to be the full string that is used. Therefore, cleaning up some code that was incorrectly using it as part of a longer string.
• Reducing the frequency of Dependabot PRs and requesting only security fixes. Currently it seems to be proposing fixes for minor version increases that have no security aspect.