Migrate PullApprove configuration from v1 to v2.

I've followed https://docs.pullapprove.com/migrating-to-v2/ to do so.

We're migrating because v2 is required to recognize GitHub review approvals as LGTM. See dropseed/pullapprove-support#98 (comment)

.pullapprove.yml

@@ -1,25 +1,26 @@
+version: 2
 approve_by_comment: true
 approve_regex: ':lgtm:|^(LGTM|lgtm)'
 reject_regex: '^Rejected'
 reset_on_push: false
-reviewers:
-  name: vitess-eng
+groups:
+  vitess-eng:
   required: 1
-  members:
-  - alainjobart
-  - alainjobart-bot
-  - AndyDiamondstein
-  - dumbunny
-  - enisoc
-  - enisoc-bot
-  - erzel
-  - erzel-bot
-  - mberlin-bot
-  - michael-berlin
-  - sougou
-  - sougou-bot
-  - thompsonja
-  - thompsonja-bot
-  - pivanof
-  - pivanof-bot
+    users:
+    - alainjobart
+    - alainjobart-bot
+    - AndyDiamondstein
+    - dumbunny
+    - enisoc
+    - enisoc-bot
+    - erzel
+    - erzel-bot
+    - mberlin-bot
+    - michael-berlin
+    - sougou
+    - sougou-bot
+    - thompsonja
+    - thompsonja-bot
+    - pivanof
+    - pivanof-bot