show system message on Jenkins header

[mawinter69]

move the system message inside the enabled block as it can be shown anyway only when it is enabled.

fixes: #71

Testing done

Manual testing

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[jonesbusy]

This change cause for me instance fail to start with JCasC configuration

    appearance:
      customHeader:
        systemMessage:
          color: lightyellow
          message: "***** platform won't be available on April 27-28th 2024, all weekend"

Caused by: io.jenkins.plugins.casc.ConfigurationAsCodeBootFailure: io.jenkins.plugins.casc.ConfiguratorException: 'color' is deprecated
at io.jenkins.plugins.casc.ConfigurationAsCode.init(ConfigurationAsCode.java:343)
... 14 more
Caused by: io.jenkins.plugins.casc.ConfiguratorException: 'color' is deprecated
at io.jenkins.plugins.casc.BaseConfigurator.configure(BaseConfigurator.java:330)
at io.jenkins.plugins.casc.BaseConfigurator.configure(BaseConfigurator.java:283)

According to the plugin release notes it's not clear there is a breaking change and what to change

[mawinter69]

It was actually #91 that introduced the breaking change, added the missing breaking change info there. Sorry for the inconvenience.