Deprecate v2

[habara-k]

Motivation

With LineBotClient / AsyncLineBotClient available as the recommended entry point, the legacy linebot (v2) modules should be explicitly marked as deprecated to guide users toward
migration.

What's changed

Deprecation annotations

• linebot/api.py: Updated all @deprecated messages on LineBotApi class and every method to point to LineBotClient instead of individual v3 API classes (e.g., MessagingApi)
• linebot/async_api.py: Same treatment for AsyncLineBotApi, pointing to AsyncLineBotClient
• linebot/models/*.py (13 files): Added @deprecated decorator to all model classes (Action, PostbackAction, FlexMessage, RichMenuSize, etc.) pointing to their
  linebot.v3.messaging / linebot.v3.webhooks equivalents
• linebot/http_client.py, linebot/async_http_client.py, linebot/aiohttp_async_http_client.py: Added @deprecated to HttpClient, RequestsHttpClient,
  AsyncHttpClient, etc.
• linebot/exceptions.py: Added @deprecated to BaseError and LineBotApiError
• linebot/constants/postback_input_option.py: Added @deprecated

MIGRATION.md

Comprehensive migration guide covering:

• Client construction (before/after)
• Import mapping (linebot.models → linebot.v3.messaging / linebot.v3.webhooks)
• Method name mapping with examples
• Error handling changes
• Full before/after echo-bot example

README.rst

• Added link to MIGRATION.md in the Version 3.x section