Update rebar and rebar3 binaries to support Erlang/OTP 25-28

[badlop]

ejabberd git repository includes the rebar and rebar3 binaries to allow compiling ejabberd with just requiring Erlang/OTP installation.

In general, those binaries are compatible with three major Erlang/OTP versions. In particular, the binaries included right now in ejabberd's master branch are compatible with Erlang 24 up to 27.

Now that Erlang/OTP 28.0-rc1 has been published, it would be nice to update the rebar binaries included in ejabberd to support Erlang 28. That is, be compatible from Erlang 25 up to 28. That would imply mentioning in the documentation how to obtain older binaries compatible with older Erlang releases. All this is very easy to do.

Right now there is some work in rebar3 to get it ready for OTP 28. Once a new rebar3 release with compatibility with OTP 28 gets published, we could update our rebar binaries accordingly.