Add property to change the CMake variable names generated with CMakeDeps

[juansblanco]

Changelog: Feature: Add new variable to CMakeDeps generator to allow changing the name used in CMake variables Docs: https://github.com/conan-io/docs/pull/XXXX

Created new property for CMakeDeps generator: cmake_legacy_variable_prefix that can be set in package_info() of the conanfile. It allows changing the prefix used when creating CMake variables instead of the package name that was currently being used.

Closes: https://github.com/conan-io/conan/issues/14788

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