Fully adopt OTel severity system

[djaglowski]

Resolves: #227

Two additional notes on this implementation:

Default

A default severity level is (still) supported. I considered allowing users to set a "default default", but there isn't a clear single point of configuration for such a setting. For now, I've just left it in place, and it can continue to be handled in the collector's internal/stanza/converter.go.

Custom Levels

Since all severity levels in this design are named, there is no need to support custom integer severity levels. The only impact of this that some mapping validation code could be removed, and some associated test cases were invalided.

[codecov]

Codecov Report

Merging #228 (6fe0737) into main (038b640) will decrease coverage by 0.2%.
The diff coverage is 91.6%.

@@           Coverage Diff           @@
##            main    #228     +/-   ##
=======================================
- Coverage   75.9%   75.7%   -0.3%     
=======================================
  Files         95      95             
  Lines       4476    4450     -26     
=======================================
- Hits        3398    3369     -29     
- Misses       749     751      +2     
- Partials     329     330      +1     

Impacted Files	Coverage Δ
logger/parser.go	47.8% <0.0%> (ø)
operator/builtin/parser/syslog/data.go	89.4% <ø> (ø)
operator/builtin/parser/syslog/syslog.go	67.8% <ø> (ø)
entry/severity.go	66.6% <100.0%> (-33.4%)	⬇️
operator/builtin/input/windows/xml.go	82.6% <100.0%> (ø)
operator/helper/severity_builder.go	85.7% <100.0%> (+0.2%)	⬆️
operator/builtin/parser/severity/severity.go	60.0% <0.0%> (-13.4%)	⬇️
operator/helper/severity.go	71.7% <0.0%> (-10.3%)	⬇️