Bump mysql-connector-java from 8.0.18 to 8.0.28 in /activiti-workflow

Bumps [mysql-connector-java](https://github.com/mysql/mysql-connector-j) from 8.0.18 to 8.0.28.
- [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@8.0.18...8.0.28)

---
updated-dependencies:
- dependency-name: mysql:mysql-connector-java
  dependency-type: direct:production
...

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Author: dependabot[bot]

activiti-workflow/pom.xml

@@ -25,7 +25,7 @@
         <springboot.version>2.3.0.RELEASE</springboot.version>
         <activiti.version>6.0.0</activiti.version>
         <swagger2.version>2.9.2</swagger2.version>
-        <mysql.version>8.0.18</mysql.version>
+        <mysql.version>8.0.28</mysql.version>
         <lombok.version>1.18.12</lombok.version>
         <mybatis.springboot.version>1.3.2</mybatis.springboot.version>
         <hutool.version>5.3.8</hutool.version>