Fix PlanarReflection support in buildsystem (used by Heightmap)

## Environment
* OS Version: 20.04 / 22.04
* Source or binary build? Source builds when creating packages in Ubuntu Focal and Jammy

## Description
When creating the Ubuntu Jammy packages using ogre-next from Ubuntu repositories, the dependencies of the package brought into the system the `libogrenextplanarreflections2.2.5` package. While Ignition does not use the PlanarReflection component directly, the heightmap feature uses it (mainly code in directory src/terrain/Terra/src/Hlms). The use of PlanarReflection is [conditionally controlled in the source code](https://github.com/ignitionrobotics/ign-rendering/blob/ign-rendering6/ogre2/src/terrain/Terra/src/Hlms/OgreHlmsTerra.cpp#L57-L59) but the buildsystem seems not to have any reference to PlanarReflection.

The Jammy package builds corresponding to version 6.2.1 (and probably lower) displays a problem with missing symbols found:
```
dpkg-shlibdeps: warning: symbol _ZNK4Ogre17PlanarReflections26_isUpdatingRenderablesHlmsEv used by debian/libignition-rendering6-ogre2/usr/lib/x86_64-linux-gnu/libignition-rendering6-ogre2.so.6.2.2~pre2 found in none of the libraries
dpkg-shlibdeps: warning: symbol _ZNK4Ogre17PlanarReflections20hasPlanarReflectionsEPKNS_10RenderableE used by debian/libignition-rendering6-ogre2/usr/lib/x86_64-linux-gnu/libignition-rendering6-ogre2.so.6.2.2~pre2 found in none of the libraries
dpkg-shlibdeps: warning: symbol _ZN4Ogre17PlanarReflections37_notifyRenderableFlushedHlmsDatablockEPNS_10RenderableE used by debian/libignition-rendering6-ogre2/usr/lib/x86_64-linux-gnu/libignition-rendering6-ogre2.so.6.2.2~pre2 found in none of the libraries
dpkg-shlibdeps: warning: symbol XDestroyWindow used by debian/libignition-rendering6-ogre2/usr/lib/x86_64-linux-gnu/libignition-rendering6-ogre2.so.6.2.2~pre2 found in none of the libraries
dpkg-shlibdeps: warning: symbol _ZNK4Ogre17PlanarReflections10getTextureEh used by debian/libignition-rendering6-ogre2/usr/lib/x86_64-linux-gnu/libignition-rendering6-ogre2.so.6.2.2~pre2 found in none of the libraries
dpkg-shlibdeps: warning: debian/libignition-rendering6-ogre2/usr/lib/x86_64-linux-gnu/ign-rendering-6/engine-plugins/libignition-rendering6-ogre2.so.6.2.2~pre2 contains an unresolvable reference to symbol _ZN4Ogre17PlanarReflections37_notifyRenderableFlushedHlmsDatablockEPNS_10RenderableE: it's probably a plugin
```
I was not able to make the simulator to crash by running on Jammy the example of `heightmap.sdf`, not sure if PlanarReflections is being loaded as a plugin or I did not run the right code to make it crash.

The current [buildsystem does not include PlanarReflections](https://github.com/ignitionrobotics/ign-rendering/blob/ign-rendering6/CMakeLists.txt#L106-L109) when looking for Ogre2 components.