Bump mysql-connector-java from 8.0.14 to 8.0.15 (#4627)

Bumps [mysql-connector-java](https://github.com/mysql/mysql-connector-j) from 8.0.14 to 8.0.15.
- [Release notes](https://github.com/mysql/mysql-connector-j/releases)
- [Changelog](https://github.com/mysql/mysql-connector-j/blob/release/8.0/CHANGES)
- [Commits](mysql/mysql-connector-j@8.0.14...8.0.15)

Signed-off-by: dependabot[bot] <support@dependabot.com>

build.gradle

@@ -120,7 +120,7 @@ dependencies {
     antlr4 'org.antlr:antlr4:4.7.2'
     compile 'org.antlr:antlr4-runtime:4.7.2'
 
-    compile 'mysql:mysql-connector-java:8.0.14'
+    compile 'mysql:mysql-connector-java:8.0.15'
 
     compile 'org.postgresql:postgresql:42.2.5'