Upgrade golangci-lint, more linters

[JoeTurki]

Description

This is the first PR of two I fixed all the issues that doesn't require refactor, ignored the ones that does.

pion/.goassets#220

[codecov]

Codecov Report

Attention: Patch coverage is 89.82036% with 17 lines in your changes missing coverage. Please review.

Project coverage is 83.70%. Comparing base (cd52e6b) to head (9f84b58).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
header_extension.go	47.61%	11 Missing ⚠️
codecs/vp9/header.go	86.66%	2 Missing ⚠️
vlaextension.go	83.33%	1 Missing and 1 partial ⚠️
codecs/vp9_packet.go	93.33%	1 Missing ⚠️
packet.go	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
- Coverage   83.71%   83.70%   -0.01%     
==========================================
  Files          24       24              
  Lines        2450     2535      +85     
==========================================
+ Hits         2051     2122      +71     
- Misses        340      354      +14     
  Partials       59       59              

Flag	Coverage Δ
go	83.70% <89.82%> (-0.01%)	⬇️
wasm	83.03% <89.22%> (+0.01%)	⬆️

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[JoeTurki]

LGTM