test: fix hash expression incompatibility for Ruby 3.4

[kenhys]

Since Ruby 3.4, evaluated Hash notation was changed.

Before:

:symbol => value

After:

symbol: value

To keep compatibility, embed evaluated hash.

It fixes the following error:

  Failure:
  test_writes_to_extracted_host_with_placeholder_replaced_in_exception_message(ElasticsearchOutputTest::HostnamePlaceholders)
  /work/fluent/plugins/fluent-plugin-elasticsearch.work/test/plugin/test_out_elasticsearch.rb:5008:in
  'ElasticsearchOutputTest::HostnamePlaceholders#test_writes_to_extracted_host_with_placeholder_replaced_in_exception_message'
  <"could not push logs to Elasticsearch
  cluster ({:host=>\"myhost-1\", :port=>9200, :scheme=>\"http\"}):
  [503] "> expected but was <"could not push logs to Elasticsearch
  cluster ({host: \"myhost-1\", port: 9200, scheme: \"http\"}): [503]
  ">

(check all that apply)

• [- ] tests added
• tests passing
• [- ] README updated (if needed)
• [-] README Table of Contents updated (if needed)
• History.md and version in gemspec are untouched
• backward compatible
• [-] feature works in elasticsearch_dynamic (not required but recommended)

[cosmo0920]

Good catch. Could you address the CI tasks to spin up Ruby 3.3 and 3.4 later, @kenhys?