Clean up gRPC API for plugins

[huseyinbabal]

We accept slice of Config object for plugin initialization, but for the simplicity, it would be better to accept single config also we have plan to spinup dedicated plugin instances. They will definitely accept single config. We need to update config part to have only single config object per plugin source/executor config group.

• Go through codebase and remove such possibility only for source plugin
• Update development guide
• Check examples in repository template

AC

• Remove deprecated fields in gRPC API + Go code related to plugin communication (remove deprecated event, there's a message field)
  • remove slice of configs in favor of a single config
    • such thing will introduce inconsistency between source and executor plugin
• Remove unused fields event.Event if there are any
• Clean up the executor config if there are some outdated fields
• Ensure tutorials, architecture, plugin docs, repository template are updated - it's about removing
• Ensure @huseyinbabal blogpost is updated with new API? Or wait until we can safely release the updated API
• Update example repository and make sure we use latest Botkube version in go.mod (and make sure it compiles)
  • wait for Clean up gRPC API for plugins #1197

[mszostok]

template repo will be updated once a new tag will be released: #1192

the repo already uses the newest API so there won't be any more work required.