doc/man7/openssl-env: sort *_PROXY environment variables, add lowercase variants

doc/man7/openssl-env: sort *_PROXY environment variables, add lowercase variants

Sort HTTP_PROXY, HTTPS_PROXY, and NO_PROXY in lexicographical order, add
mentions of the lowercase variants of the variables (as they are already
mentioned in OSSL_HTTP_parse_url(3)), in the same order, despite
it is not a lexicographical one, do minimise confusion.

Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28025)