Bump mariadb-java-client from 2.7.0 to 2.7.3 in /modules/mariadb (#4090)

Bumps [mariadb-java-client](https://github.com/mariadb-corporation/mariadb-connector-j) from 2.7.0 to 2.7.3.
- [Release notes](https://github.com/mariadb-corporation/mariadb-connector-j/releases)
- [Changelog](https://github.com/mariadb-corporation/mariadb-connector-j/blob/2.7.3/CHANGELOG.md)
- [Commits](mariadb-corporation/mariadb-connector-j@2.7.0...2.7.3)

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

modules/mariadb/build.gradle

@@ -10,7 +10,7 @@ dependencies {
     compileOnly 'org.mariadb:r2dbc-mariadb:1.0.0'
 
     testCompile project(':jdbc-test')
-    testCompile 'org.mariadb.jdbc:mariadb-java-client:2.7.0'
+    testCompile 'org.mariadb.jdbc:mariadb-java-client:2.7.3'
 
     testCompile testFixtures(project(':r2dbc'))
     testCompile 'org.mariadb:r2dbc-mariadb:1.0.0'