Update gatsby monorepo

Author: renovate[bot]

package.json

@@ -16,26 +16,26 @@
     "bulma": "0.9.3",
     "classnames": "2.3.1",
     "feed": "4.2.2",
-    "gatsby": "3.8.1",
+    "gatsby": "3.15.0",
     "gatsby-background-image": "1.5.3",
-    "gatsby-image": "3.8.0",
-    "gatsby-plugin-catch-links": "3.8.0",
-    "gatsby-plugin-google-analytics": "3.8.0",
-    "gatsby-plugin-image": "1.8.0",
-    "gatsby-plugin-manifest": "3.8.0",
+    "gatsby-image": "3.11.0",
+    "gatsby-plugin-catch-links": "3.15.0",
+    "gatsby-plugin-google-analytics": "3.15.0",
+    "gatsby-plugin-image": "1.15.0",
+    "gatsby-plugin-manifest": "3.15.0",
     "gatsby-plugin-mdx": "2.8.0",
-    "gatsby-plugin-page-creator": "3.8.0",
-    "gatsby-plugin-react-helmet": "4.8.0",
+    "gatsby-plugin-page-creator": "3.15.0",
+    "gatsby-plugin-react-helmet": "4.15.0",
     "gatsby-plugin-robots-txt": "1.6.8",
-    "gatsby-plugin-sass": "4.8.0",
-    "gatsby-plugin-sharp": "3.8.0",
-    "gatsby-plugin-sitemap": "4.4.0",
-    "gatsby-remark-copy-linked-files": "4.5.0",
-    "gatsby-remark-images": "5.5.0",
-    "gatsby-source-contentful": "5.8.0",
-    "gatsby-source-filesystem": "3.8.0",
+    "gatsby-plugin-sass": "4.15.0",
+    "gatsby-plugin-sharp": "3.15.0",
+    "gatsby-plugin-sitemap": "4.11.0",
+    "gatsby-remark-copy-linked-files": "4.12.0",
+    "gatsby-remark-images": "5.12.0",
+    "gatsby-source-contentful": "5.15.0",
+    "gatsby-source-filesystem": "3.15.0",
     "gatsby-transformer-remark": "4.5.0",
-    "gatsby-transformer-sharp": "3.8.0",
+    "gatsby-transformer-sharp": "3.15.0",
     "gensync": "1.0.0-beta.2",
     "immutability-helper": "3.1.1",
     "lodash": "4.17.21",