doc: use the documented =item markers

doc: use the documented =item markers

The generated lists[1] look weird when using a dash as the list item
character. Perlpod documents[2] '*' for unordered lists and '1.' (note
the period) for ordered lists. Use these characters instead.

[1] e.g. https://www.openssl.org/docs/manmaster/man7/migration_guide.html#New-Algorithms
[2] https://perldoc.perl.org/perlpod

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16190)