Move MP level to 7.1 on the parent pom

[Emily-Jiang]

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Additional context

[arjav-desai]

Also, We may want to make sure that https://github.com/langchain4j/langchain4j-cdi/blob/main/langchain4j-cdi-mp/pom.xml is also aligned with MP 7.1

[arjav-desai]

None of the implementation except liberty currently supports 7.1 so we should keep MP to 6.1 for 1.0 release at least.