Update mysql2, json, to fix ruby 2.7 warnings (rails#37967)

* Update mysql2 to the latest version

Fixes keyword arguments warning:
brianmario/mysql2/pull/1084

Fixes taint mechanism deprecation warning:
brianmario/mysql2@785969f

* Update json to the latest version

Fixes warning from trying to access an uninitialized ivar, and
keyword arguments warning:

flori/json/commit/de14dea34074558dc671d7edc71513f0216ce21d
flori/json/commit/19da336333e63dc9dde7baea47b179e162b7568e

Author: utilum

Gemfile.lock

@@ -290,8 +290,8 @@ GEM
     jdbc-postgres (42.2.6)
     jdbc-sqlite3 (3.28.0)
     jmespath (1.4.0)
-    json (2.2.0)
-    json (2.2.0-java)
+    json (2.3.0)
+    json (2.3.0-java)
     jwt (2.2.1)
     kindlerb (1.2.0)
       mustache
@@ -335,9 +335,9 @@ GEM
     multipart-post (2.1.1)
     mustache (1.1.0)
     mustermann (1.0.3)
-    mysql2 (0.5.2)
-    mysql2 (0.5.2-x64-mingw32)
-    mysql2 (0.5.2-x86-mingw32)
+    mysql2 (0.5.3)
+    mysql2 (0.5.3-x64-mingw32)
+    mysql2 (0.5.3-x86-mingw32)
     nio4r (2.5.2)
     nio4r (2.5.2-java)
     nokogiri (1.10.4)