Add Store Channels to FinderUtil

[Mikusch]

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Description

This PR adds JDA 4's Store Channels to the FinderUtil class and removes the genericTextChannelSearch, genericVoiceChannelSearch, and genericCategorySearch methods in favor of one generic channel search method.

[Mikusch]

findStoreChannels(String, JDA) does not query a ShardManager if it is present at the moment. This is due to ShardManager missing the related cache access methods as of version 4.BETA.0_25.

I only realized that after creating this PR (I simply thought store channels weren't cached in the ShardManager for some odd reason).

I've opened another PR over at discord-jda/JDA#1060 to add those methods in.