Migrating from Gun 2.0 to 2.1

Gun 2.1 contains a small security improvement for the HTTP/2 protocol, as well as includes a small number of fixes and improvements.

Gun 2.1 requires Erlang/OTP 22.0 or greater.

Features added

• A new HTTP/2 option max_fragmented_header_block_size has been added to limit the size of header blocks that are sent over multiple HEADERS and CONTINUATION frames.
• Update Cowlib to 2.13.0.

Bugs fixed

• Gun will no longer configure the NPN TLS extension, which has long been replaced by ALPN. NPN is not compatible with TLS 1.3.
• Gun will no longer crash when TLS connections close very early in the connection's life time.