deps: Upgrade ESLint to ^6.8.0, and related deps.

We're not quite ready for ESLint 7 because several of our plugins
don't support it yet. Fair enough; 6.8.0 is the latest on ESLint 6.

Done with the following commands:

```
yarn upgrade --latest \
  eslint@^6.8.0 \
  eslint-plugin-flowtype@^4.7.0 \
  eslint-plugin-jest@^22.21.0 \
  $(
    tools/deps | grep eslint \
    | grep -vx \
        -e eslint \
        -e eslint-plugin-flowtype \
        -e eslint-plugin-jest
  )
```

then

```
yarn add eslint-plugin-react-hooks@^4.0.8
```

- Any later `eslint-plugin-flowtype`, and we'd have to be using
  ESLint 7.

- Any later `eslint-plugin-jest`, and one of its rather distant
  dependencies declares `typescript` as a peer dependency.

- `eslint-plugin-react-hooks` is a peer dependency of the new
  version of `eslint-config-airbnb`, with ^4 being part of the
  version range. 4.0.8 is the latest.

Then `yarn yarn-deduplicate && yarn` as prompted by
`tools/test deps`.

Fixes: zulip#4120

package.json

@@ -42,6 +42,7 @@
     "blueimp-md5": "^2.10.0",
     "color": "^3.0.0",
     "date-fns": "^1.29.0",
+    "eslint-plugin-react-hooks": "^4.0.8",
     "expo-apple-authentication": "^2.1.1",
     "expo-application": "^2.1.1",
     "expo-screen-orientation": "^1.0.0",
@@ -98,15 +99,15 @@
     "babel-eslint": "^10.0.3",
     "babel-plugin-transform-remove-console": "^6.9.4",
     "deep-freeze": "^0.0.1",
-    "eslint": "^5.16.0",
-    "eslint-config-airbnb": "^17.1.1",
-    "eslint-config-prettier": "^6.0.0",
-    "eslint-plugin-flowtype": "^3.9.0",
-    "eslint-plugin-import": "^2.18.1",
-    "eslint-plugin-jest": "^22.11.1",
-    "eslint-plugin-jsx-a11y": "^6.2.3",
-    "eslint-plugin-prettier": "^3.1.0",
-    "eslint-plugin-react": "^7.14.2",
+    "eslint": "^6.8.0",
+    "eslint-config-airbnb": "^18.2.0",
+    "eslint-config-prettier": "^6.11.0",
+    "eslint-plugin-flowtype": "^4.7.0",
+    "eslint-plugin-import": "^2.22.0",
+    "eslint-plugin-jest": "^22.21.0",
+    "eslint-plugin-jsx-a11y": "^6.3.1",
+    "eslint-plugin-prettier": "^3.1.4",
+    "eslint-plugin-react": "^7.20.5",
     "flow-bin": "^0.105.0",
     "flow-coverage-report": "^0.6.0",
     "flow-typed": "^2.4.0",
@@ -120,7 +121,7 @@
     "lolex": "^5.1.1",
     "metro-react-native-babel-preset": "^0.56.0",
     "prettier": "^1.18.2",
-    "prettier-eslint": "^9.0.0",
+    "prettier-eslint": "^11.0.0",
     "prettier-eslint-cli": "^5.0.0",
     "prop-types": "^15.7.2",
     "react-dom": "16.9.0",