Release langchain==0.2.6

Changes since langchain==0.2.5

langchain: release 0.2.6 (#23426)
langchain[patch]: update removal target for LLMChain (#23373)
core[minor]: Update pgvector transalor for langchain_postgres (#23217)
langchain: fix systax warning in create_json_chat_agent (#23253)
community[patch]: restore compatibility with SQLAlchemy 1.x (#22546)
langchain[small]: Change type to BasePromptTemplate (#23083)
langchain[patch]: fix OutputType of OutputParsers and fix legacy API in OutputParsers (#19792)
docs: Fix URL formatting in deprecation warnings (#23075)
core[patch],community[patch],langchain[patch]: tenacity dependency to version >=8.1.0,<8.4.0 (#22973)
langchain: add id_key option to EnsembleRetriever for metadata-based document merging (#22950)
langchain[patch]: add tool messages formatter for tool calling agent (#22849)