[DO NOT MERGE] minsev: Fix to filter out only in SeverityTrace1..SeverityFatal4 severity range

[pellared]

Per open-telemetry/opentelemetry-specification#4541

From https://github.com/open-telemetry/opentelemetry-specification/blob/bdcf53d55285ae42339f37908a24ba6f3ac167cb/specification/logs/data-model.md?plain=1#L418-L423:

In the contexts where severity participates in less-than / greater-than
comparisons SeverityNumber field should be used. SeverityNumber can be
compared to another SeverityNumber or to numbers in the 1..24 range (or to the
corresponding short names).

Currently the spec only describes how to compare values between 1 and 24.
All other values have undefined meaning.

The main reason is to NOT filter out records with log.SeverityUndefined severity which can be a common scenario.

I also did my best to improve the docs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.2%. Comparing base (bc2c50a) to head (216ad10).
Report is 113 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7424   +/-   ##
=====================================
  Coverage   82.2%   82.2%           
=====================================
  Files        205     205           
  Lines      17949   17952    +3     
=====================================
+ Hits       14768   14774    +6     
+ Misses      2743    2741    -2     
+ Partials     438     437    -1     

Files with missing lines	Coverage Δ
processors/minsev/minsev.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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[pellared]

Marking as blocked by specification as I do not want us to go back and forth until:

• Fix SeverityProcessor example opentelemetry-specification#4541

and maybe

• Document how consumers filtering logs on severity should treat unset or unspecified (0) severity opentelemetry-specification#4540

are resolved.

[pellared]

From the SIG meetings discussions it seems that most prefer our current implementation: open-telemetry/opentelemetry-specification#4541 (comment).

I will extract the Go docs improvements to a separate PR.

[pellared]

Closed per open-telemetry/opentelemetry-specification#4552