WebDisplay: RGeomViewer is no longer experimental

[andresailer]

LCG nightly builds:

/build/jenkins/workspace/lcg_nightly_pipeline/build/frameworks/DD4hep-master/src/DD4hep/master/DDEve/root7/WebDisplay.cpp:22:41: error: 'RGeomViewer' in namespace 'ROOT::Experimental' does not name a type
   22 | using GEOM_VIEWER = ROOT::Experimental::RGeomViewer;
      |                                         ^~~~~~~~~~~
/build/jenkins/workspace/lcg_nightly_pipeline/build/frameworks/DD4hep-master/src/DD4hep/master/DDEve/root7/WebDisplay.cpp: In function 'long int webdisplay(dd4hep::Detector&, int, char**)':
/build/jenkins/workspace/lcg_nightly_pipeline/build/frameworks/DD4hep-master/src/DD4hep/master/DDEve/root7/WebDisplay.cpp:84:36: error: 'GEOM_VIEWER' was not declared in this scope
   84 |     auto viewer = std::make_shared<GEOM_VIEWER>(&mgr);
      |                                    ^~~~~~~~~~~
/build/jenkins/workspace/lcg_nightly_pipeline/build/frameworks/DD4hep-master/src/DD4hep/master/DDEve/root7/WebDisplay.cpp:84:48: error: no matching function for call to 'make_shared<<expression error> >(TGeoManager*)'
   84 |     auto viewer = std::make_shared<GEOM_VIEWER>(&mgr);

BEGINRELEASENOTES

• DDEve Webdisplay: fix issue with ROOT master with RGeomViewer no longer being in the experimental namespace.

ENDRELEASENOTES

[github-actions]

Test Results

    4 files      4 suites   4h 32m 21s ⏱️
355 tests 355 ✔️ 0 💤 0 ❌
705 runs  705 ✔️ 0 💤 0 ❌

Results for commit 8952545.

[andresailer]

dev3 tests are failing because that still has the ROOT from yesterday, but root-project/root#13510 was merged yesterday, so we are failing to compile DD4hep in the dev3 nightlies. Merging this PR and tomorrow the tests should work again.