rename to_syslog_severity to parse_syslog_severity

[JeanMertz]

This one is incorrectly named. It behaves similar to other parse_* functions (take in a string, try to parse it to the relevant type, error if it can't), but is named as a to_* function.

[binarylogic]

It should be named parse_syslog_level, which is less obvious that it's returning a severity number. I am personally a fan of keeping it named to_syslog_severity and have it accept a string and int:

to_syslog_severity("info") => 2
to_syslog_severity(2) => 2

[jszwedko]

Discussed and decided to go with parse_syslog_severity to match other parsing functions.

[jszwedko]

Might be a little bit of extra work here if it is the first function we are deprecating and aliasing.