Updating notebook pip installs after v0.11 release (quantumlib#4102)

Updating notebook `pip install`s after v0.11 release. 
- Changed all `pip install cirq --pre` to `pip install cirq`. 
- Removed pre-release notices. 
- Removed pre-release skip list from isolated_notebook_tests.py. 
- cirq[contrib] is now cirq-core[contrib] so I updated notebooks and docs
- educators/chemistry.ipynb testing is skipped currently until we upgrade OpenFermion to the new Cirq (it is still using `cirq.TrialResult`, that was removed in v0.11)

Author: balopat

cirq/contrib/quimb/Cirq-to-Tensor-Networks.ipynb

@@ -18,9 +18,9 @@
     "try:\n",
     "    import quimb\n",
     "except ImportError:\n",
-    "    print(\"installing cirq[contrib]...\")\n",
-    "    !pip install --quiet cirq[contrib]\n",
-    "    print(\"installed cirq[contrib].\")\n",
+    "    print(\"installing cirq-core[contrib]...\")\n",
+    "    !pip install --quiet cirq-core[contrib]\n",
+    "    print(\"installed cirq-core[contrib].\")\n",
     "        "
    ]
   },
@@ -387,4 +387,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 4
-}
+}

cirq/contrib/quimb/Contract-a-Grid-Circuit.ipynb

@@ -18,9 +18,9 @@
     "try:\n",
     "    import quimb\n",
     "except ImportError:\n",
-    "    print(\"installing cirq[contrib]...\")\n",
-    "    !pip install --quiet cirq[contrib]\n",
-    "    print(\"installed cirq[contrib].\")    "
+    "    print(\"installing cirq-core[contrib]...\")\n",
+    "    !pip install --quiet cirq-core[contrib]\n",
+    "    print(\"installed cirq-core[contrib].\")"
    ]
   },
   {
@@ -425,4 +425,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 4
-}
+}