lint: Upgrade ESLint to ^6.8.0.

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.

Apart from that, I was aware of some specific minimum versions of
things we'll need, so I just made those tweaks on that basis:

- `prettier-eslint` at 10.1.1 is the minimum that I'm confident has
  full ESLint 6 support; see my comment there at
  prettier/prettier-eslint#338 (comment).

- `eslint-config-airbnb` at 18.1.0, I'd discovered earlier, has
  6.8.0 as a peer dep. I checked to see if 18.1.0 is the latest
  version, and it is.

Otherwise, I just followed the trail of peer dependency warnings
until I could run `rm -rf node_modules && yarn` without any
warnings. It's likely that this would have given the same outcome
for `prettier-eslint` and `eslint-config-airbnb`, but it's not what
I did for those.

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

Fixes: zulip#4120

ios/Podfile.lock

@@ -399,6 +399,6 @@ SPEC CHECKSUMS:
   UMTaskManagerInterface: 1e70fe58b872355f0ecb44fb81bb1a16484047f0
   yoga: 5079887aa3e4c62142d6bcee493022643ee4d730
 
-PODFILE CHECKSUM: 35e31db914557542fa277c2ec99e983927f39658
+PODFILE CHECKSUM: 0ad43557c2cc3f3ba3bbf1b11cd23d2ac5ba6e28
 
 COCOAPODS: 1.9.1

package.json

@@ -96,15 +96,16 @@
     "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": "^6.8.0",
+    "eslint-config-airbnb": "^18.1.0",
     "eslint-config-prettier": "^6.0.0",
     "eslint-plugin-flowtype": "^3.9.0",
-    "eslint-plugin-import": "^2.18.1",
+    "eslint-plugin-import": "^2.20.1",
     "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-plugin-react": "^7.19.0",
+    "eslint-plugin-react-hooks": "2.5.0",
     "flow-bin": "^0.98.0",
     "flow-coverage-report": "^0.6.0",
     "flow-typed": "^2.4.0",
@@ -117,7 +118,7 @@
     "lolex": "^5.1.1",
     "metro-react-native-babel-preset": "^0.56.0",
     "prettier": "^1.18.2",
-    "prettier-eslint": "^9.0.0",
+    "prettier-eslint": "^10.1.1",
     "prettier-eslint-cli": "^5.0.0",
     "prop-types": "^15.7.2",
     "react-native-cli": "^2.0.1",