[MNG-6728] toolchain.xml should support absense of version and/or vendor in the configuraion

**[Hüseyin Kartal](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=hsyn)** opened **[MNG-6728](https://issues.apache.org/jira/browse/MNG-6728?redirect=false)** and commented

When a toolchain.xml is configured as follow:

```

jdk

8
oracle


${env.JDK_HOME_8}



jdk

12
oracle


${env.JDK_HOME_12}



jdk

${env.JAVA_HOME}



```

The last jdk configuration without version and vendor should match any requested version and/or vendor.

This would allow us to define a fallback toolchain.

The idea behind this improvement is that we don't want the toolchain plugin to enforce a jdk, this we would do with the enforcer plugin. We want to use the toolchain plugin just for supply the best JDK. We also want reduce the maintainance of the toolchain.xml on our different development and cicd systems.

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**Affects:** 3.6.1