Update go-syslog to accept non-UTF8 encoding in syslog message

[bastjan]

What this PR does / why we need it:

Related to influxdata/go-syslog#35.

Stops promtail from dropping syslog connections if the message does contain characters in a non-UTF8 encoding. RFC5424 explicitly allows non-UTF8 messages.

File target passes non-UTF8 text as-is too.

Which issue(s) this PR fixes:

Fixes #1783

Special notes for your reviewer:

Casting to *rfc5424.SyslogMessage is now necessary since since go-syslog could now support rfc3164 (BSD syslog) if enabled.

Checklist

• Documentation added
• Tests updated

[codecov-commenter]

Codecov Report

Merging #2127 into master will increase coverage by 0.11%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master    #2127      +/-   ##
==========================================
+ Coverage   61.25%   61.37%   +0.11%     
==========================================
  Files         146      146              
  Lines       11196    11197       +1     
==========================================
+ Hits         6858     6872      +14     
+ Misses       3793     3777      -16     
- Partials      545      548       +3     

Impacted Files	Coverage Δ
pkg/promtail/targets/syslogparser/syslogparser.go	100.00% <ø> (ø)
pkg/promtail/targets/syslogtarget.go	74.83% <81.81%> (+0.16%)	⬆️
pkg/promtail/targets/tailer.go	76.13% <0.00%> (-2.28%)	⬇️
pkg/promtail/positions/positions.go	60.71% <0.00%> (+13.39%)	⬆️

[cyriltovena]

LGTM Nice work !

Thank you Sebastian for this !