3.6.0

This is the first important release to support OTP-21.0 officially, rather than just release candidates. There are some new features:

• upgrades to relx to 3.25.0 introduce support for OTP-21's 'sys.config.src' files, which allow automatic replacement of variables in sys.config files, without needing to use set RELX_REPLACE_OS_VARS, and without having to wrap them in strings to work. It also contains a bunch of bugfixes.
• Introduce support of del operations for overrides, which allow to unset specific values from lists such as erl_opts in dependencies
• Add hooks to the upgrade command
• Add --start-clean flag to rebar3 shell

There are also plenty of bugfixes:

• Bump cth_readable for OTP-21 compat
• upgrade bbmustache to 1.5.0
• run compile provider in default namespace from bare compiler, fixing some build issues in mix builds
• Resolve string vs list crashing rebar3 erl_first_files
• Create ERLANG_LIB_*_erl_interface environment variables only if erl_interface exists
• Auto-boot main app in OTP app project templates
• Use maps for child spec example in templates