Bump mysql-connector-java from 8.0.21 to 8.0.28 in /demo-mybatis-plus

Bumps [mysql-connector-java](https://github.com/mysql/mysql-connector-j) from 8.0.21 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@8.0.21...8.0.28)

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

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Author: dependabot[bot]

demo-mybatis-plus/pom.xml

@@ -25,7 +25,7 @@
         <maven.compiler.target>1.8</maven.compiler.target>
         <java.version>1.8</java.version>
         <springboot.version>2.5.1</springboot.version>
-        <mysql.version>8.0.21</mysql.version>
+        <mysql.version>8.0.28</mysql.version>
         <swagger2.version>2.9.2</swagger2.version>
         <mybatis-plus.version>3.4.0</mybatis-plus.version>
         <hutool.version>5.4.1</hutool.version>