Define minimum C++ version using ROOT_CXX_STANDARD

[denproc]

Changes in this pull request

Adjusted CMake recipe to define minimum C++ version using ROOT_CXX_STANDARD.

Testing performed

The definition of the build variables has been tested on MacOS for root==6.28,6.30,6.32.
However, I could not validate if it builds successfully due to conflict of dependencies between root, STIR, and libparallelproj=2 (conda installations).

Related issues

Fixes #1677

Checklist before requesting a review

• I have performed a self-review of my code
• [] The code builds and runs on my machine
• [] documentation/release_XXX.md has been updated with any functionality change (if applicable)

Contribution Notes

Please tick the following:

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in STIR (the Work) under the terms and conditions of the Apache-2.0 License.

[codacy-production]

Up to standards ✅

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