Release langchain-core==0.2.1

core[patch]: Release 0.2.1 (#22003)
core[patch]: Add unit test for RunnableGenerator for eventstream v2 (#21990)
core[patch]: In astream_events(version=v2) tap output of root run (#21977)
docs: set default anthropic model (#21988)
core[patch]: Fix NPE in function_calling._get_python_function_required_args (#21863)
Fix base.py typo (#21862)
docs: lcel how to and cheatsheet (#21851)
core: Tap output of sync iterators for astream_events (#21842)
core[patch]: Check if event loop is closed in memory stream (#21841)