Add Online/v7r6

data/stacks/Online/v7r6.yml

+name: Online/v7r6
+type: lhcbcmake
+
+projects:
+  Gaudi: v35r2
+  Detector: v0r7
+  LHCb: v52r0
+  Online: v7r6
+
+toolchain:
+  type: LCG
+  version: 97a
+
+platforms:
+  - x86_64-centos7-gcc9-opt
+  - x86_64-centos7-gcc9-dbg
+  - x86_64-centos7-gcc9-do0
+  - x86_64-centos7-clang8-opt
+  - x86_64-centos7-clang8-dbg
+  - x86_64+avx2+fma-centos7-gcc9-opt
+
+tags:
+  - Run3