Release langchain==0.2.7

Changes since langchain==0.2.6

community: release 0.2.7 (#23993)
docs: rm discord (#23985)
langchain[patch]: deprecate QAGenerationChain (#23730)
langchain[patch]: deprecate AnalyzeDocumentChain (#23769)
langchain: update declarative_base import (#20056)
infra: update mypy 1.10, ruff 0.5 (#23721)
langchain[patch]: deprecate ConversationChain (#23504)
langchain: docstrings in agents root (#23561)
langchain: docstrings agents nested (#23598)
langchain[minor]: fix comment typo (#23564)
langchain[patch]: update agent and chains modules root_validators (#23256)
Add a conversation memory that combines a (optionally persistent) vectorstore history with a token buffer (#22155)