fix(starters): update react monorepo to ^16.8.6 (#12910)

This PR contains the following updates:

| Package | Type | Update | Change | References |
|---|---|---|---|---|
| react | dependencies | patch | [`^16.8.5` -> `^16.8.6`](https://diff.intrinsic.com/react/16.8.5/16.8.6) | [homepage](https://reactjs.org/), [source](https://github.com/facebook/react/tree/HEAD/packages/react) |
| react-dom | dependencies | patch | [`^16.8.5` -> `^16.8.6`](https://diff.intrinsic.com/react-dom/16.8.5/16.8.6) | [homepage](https://reactjs.org/), [source](https://github.com/facebook/react/tree/HEAD/packages/react-dom) |

---

### Release Notes

<details>
<summary>facebook/react</summary>

### [`v16.8.6`](https://github.com/facebook/react/blob/master/CHANGELOG.md#&#8203;1686-March-27-2019)

[Compare Source](https://github.com/facebook/react/compare/v16.8.5...v16.8.6)

##### React DOM

-   Fix an incorrect bailout in `useReducer()`. ([@&#8203;acdlite](https://github.com/acdlite) in [#&#8203;15124](https://github.com/facebook/react/pull/15124))
-   Fix iframe warnings in Safari DevTools. ([@&#8203;renanvalentin](https://github.com/renanvalentin) in [#&#8203;15099](https://github.com/facebook/react/pull/15099))
-   Warn if `contextType` is set to `Context.Consumer` instead of `Context`. ([@&#8203;aweary](https://github.com/aweary) in [#&#8203;14831](https://github.com/facebook/react/pull/14831))
-   Warn if `contextType` is set to invalid values. ([@&#8203;gaearon](https://github.com/gaearon) in [#&#8203;15142](https://github.com/facebook/react/pull/15142))

</details>

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---

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starters/blog/package.json

@@ -26,8 +26,8 @@
     "gatsby-transformer-remark": "^2.3.8",
     "gatsby-transformer-sharp": "^2.1.17",
     "prismjs": "^1.16.0",
-    "react": "^16.8.5",
-    "react-dom": "^16.8.5",
+    "react": "^16.8.6",
+    "react-dom": "^16.8.6",
     "react-helmet": "^5.2.0",
     "react-typography": "^0.16.19",
     "typeface-merriweather": "0.0.72",

starters/default/package.json

@@ -14,8 +14,8 @@
     "gatsby-source-filesystem": "^2.0.28",
     "gatsby-transformer-sharp": "^2.1.17",
     "prop-types": "^15.7.2",
-    "react": "^16.8.5",
-    "react-dom": "^16.8.5",
+    "react": "^16.8.6",
+    "react-dom": "^16.8.6",
     "react-helmet": "^5.2.0"
   },
   "devDependencies": {

starters/hello-world/package.json

@@ -14,8 +14,8 @@
   },
   "dependencies": {
     "gatsby": "^2.3.3",
-    "react": "^16.8.5",
-    "react-dom": "^16.8.5"
+    "react": "^16.8.6",
+    "react-dom": "^16.8.6"
   },
   "devDependencies": {
     "prettier": "^1.16.4"