Simplify usage for older GDAL by using CMake's supplied find module

[Ryanf55]

What does this PR do?

• Recommend using CMake's supplied find module if using older GDAL versions
• Remove broken recommendation for creating a GDAL::GDAL target

Note - Ubuntu 20 comes with CMake 3.16, so this should cover all non-EOL OS's that people will generally be using. We intentionally ignore the edge cases of custom find modules because we will assume people writing those know what they are doing.

What are related issues/pull requests?

https://github.com/OSGeo/gdal/pull/9714#discussion_r1575040858

Tasklist

• [ ] Make sure code is correctly formatted (cf pre-commit configuration)
• [ ] Add test case(s)
• [ ] Add documentation
• [ ] Updated Python API documentation (swig/include/python/docs/)
• [ ] Review
• [ ] Adjust for comments
• [ ] All CI builds and checks have passed

Environment

Provide environment details, if relevant:

• OS: Ubuntu 22 with GDAL installed through APT
• Compiler: G++11

[Ryanf55]

@dg0yt Can you please review this?