Use use module name to obtain module prefix in RedisRepositoryConfigurationExtension

[junghoon-vans]

In my opinion, returning the module name directly is not a good idea,
so I modified it to handle it by declaring a constant instead.
Also, instead of returning a value directly from getModulePrefix, we can proxies it to this.getModuleIdentifier.

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[mp911de]

With a smarter defaulting in KeyValueRepositoryConfigurationExtension (spring-projects/spring-data-keyvalue#542) we were able to actually remove a bit of code. Thank you for your contribution, that's polished, merged, and backported now.