Migrating from Cowboy 2.11 to 2.12

Cowboy 2.12 contains a small security improvement for the HTTP/2 protocol.

Cowboy 2.12 requires Erlang/OTP 24.0 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.