Replace deprecated JSON.fast_generate with JSON.generate

[pascaltozzi]

What does this pull request do?

This is a non-functional refactor with no behavioral changes.

As of version 2.11.0 of the json gem, JSON.fast_generate has been deprecated because it provides no measurable performance benefit over JSON.generate.

Why is it important?

It remove deprecation warnings and maintain forward compatibility.

Checklist

• I have signed the Contributor License Agreement.
• My code follows the style guidelines of this project (See .rubocop.yml)
• I have rebased my changes on top of the latest main branch
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have made corresponding changes to the documentation
• I have updated CHANGELOG.asciidoc
• I have updated supported-technologies.asciidoc
• Added an API method or config option? Document in which version this will be introduced

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[saiqulhaq-hh]

I just monkey patched my app because this one is not merged yet

if ElasticAPM::VERSION == '4.8.0'
    module ElasticAPM
      module Transport
        class Connection
          def initialize(config)
            @config = config
            @metadata = JSON.generate(
              Serializers::MetadataSerializer.new(config).build(
                Metadata.new(config)
              )
            )
            @url = "#{config.server_url}/intake/v2/events"
            @mutex = Mutex.new
          end
        end
      end
    end
  end