Fix NameError in v2 push_message due to missing VERSION constant

[tatematsu-k]

Summary

When using Line::Bot::V2::MessagingApi::ApiClient#push_message_with_http_info, a NameError occurs due to an uninitialized constant Line::Bot::V2::VERSION.

Reproduction Steps

1. Use the line-bot-api gem with commit ref 5bc759e0a38f4eec5715a5504463147cdffec945
2. Execute the following script:

   # Gemfile
   gem "line-bot-api", git: "https://github.com/line/line-bot-sdk-ruby.git", ref: "5bc759e0a38f4eec5715a5504463147cdffec945"

   line_retry_key = SecureRandom.uuid
   push_message_request = {
     to: "U123456789",
     messages: [{ ... }]
   }
   client = Line::Bot::V2::MessagingApi::ApiClient.new(channel_access_token: ENV["LINE_MESSAGING_CHANNEL_TOKEN"])
   response, status_code = client.push_message_with_http_info(push_message_request:, x_line_retry_key: line_retry_key)

3. The following error occurs:

'Class#new': uninitialized constant Line::Bot::V2::VERSION (NameError)

          @http_headers = { 'User-Agent' => "LINE-BotSDK-Ruby/#{Line::Bot::V2::VERSION}" }.merge(http_headers)
                                                                             ^^^^^^^^^
	# ... backtrace for Rails Application

app(dev)> Line::Bot::V2::VERSION
(app):17:in '<main>': uninitialized constant Line::Bot::V2::VERSION (NameError)

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