COIN/QtCreator: Add machine for Qt 5 based build

Since we still support Qt 5 based builds of Qt Creator, we'd like
to have that combination as part of the precheck.

Pick-to: 6.3 6.2
Change-Id: I8f752f6e2aff4bebd644a87a1f51255ca482d337
Reviewed-by: Antti Kokko <antti.kokko@qt.io>

coin/platform_configs/qt-creator.yaml

@@ -21,6 +21,12 @@ Configurations:
     Features: ['Packaging', 'Sccache', 'DebugAndRelease', 'DisableTests']
     Configure arguments: ''
     Environment variables: []
+-
+    Template: 'qtci-linux-RHEL-8.4-x86_64-50'
+    Compiler: 'GCC'
+    Features: ['Qt5', 'Sccache', 'DebugAndRelease', 'DisableTests']
+    Configure arguments: ''
+    Environment variables: []
 -
     Template: 'qtci-macos-10.14-x86_64'
     Compiler: 'Clang'