So from the following code:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.3.4</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>
 ...
<packaging>jar</packaging>
<properties>
    <java.version>21</java.version>
    <maven.compiler.source>23</maven.compiler.source>
    <maven.compiler.target>23</maven.compiler.target>

The upgrade is definitely persisted in the XML. I'm interested to know what the Heroku server config is. It's possible there is a mismatch between your local config and your remote config.

Also, did you notice any differences in your lock file?