Bump mysql-connector-java from 8.0.20 to 8.0.21 in /modules/mysql (#3045)

Bumps [mysql-connector-java](https://github.com/mysql/mysql-connector-j) from 8.0.20 to 8.0.21.
- [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@8.0.20...8.0.21)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Richard North <[email protected]>

Author: dependabot[bot]

modules/mysql/build.gradle

@@ -10,7 +10,7 @@ dependencies {
     compileOnly 'dev.miku:r2dbc-mysql:0.8.1.RELEASE'
 
     testCompile project(':jdbc-test')
-    testCompile 'mysql:mysql-connector-java:8.0.20'
+    testCompile 'mysql:mysql-connector-java:8.0.21'
 
     testCompile testFixtures(project(':r2dbc'))
     testCompile 'dev.miku:r2dbc-mysql:0.8.1.RELEASE'