MEDIUM: mailers: warn about deprecated legacy mailers

MEDIUM: mailers: warn about deprecated legacy mailers

As mentioned in 2.8 announce on the mailing list [1] and on the wiki [2],
use of legacy mailers is now deprecated and will not be supported anymore
starting with version 3.3. Use of Lua script (AKA Lua mailers) is now
encouraged (and fully supported since 2.8) for this purpose, as it offers
more flexibility (e.g: alerts can be customized) and is more future-proof.

Configurations relying on legacy mailers will now raise a warning.

Users willing to keep their existing mailers config in a working state
should simply add the following line to their global section:

   # mailers.lua file as provided in the git repository
   # adjust path as needed
   lua-load examples/lua/mailers.lua

[1]: https://www.mail-archive.com/haproxy@formilux.org/msg43600.html
[2]: https://github.com/haproxy/wiki/wiki/Breaking-changes