additional badges, fix w3c interop CI and correct the requirements

[tsloughter]

I can break this up if needed but figured they were all pretty uncontroversial changes, with the possible exception of the requirements, but I based that on what the CI is running.

[codecov]

Codecov Report

Merging #343 (9b59e3c) into main (f12732d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #343   +/-   ##
=======================================
  Coverage   33.33%   33.33%           
=======================================
  Files          56       56           
  Lines        4563     4563           
=======================================
  Hits         1521     1521           
  Misses       3042     3042           

Flag	Coverage Δ
api	67.28% <ø> (ø)
elixir	16.44% <ø> (ø)
erlang	33.27% <ø> (ø)
exporter	21.47% <ø> (ø)
sdk	77.13% <ø> (ø)
zipkin	2.59% <ø> (ø)

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[ferd]

Approved but the CI spec seems wrong based on test results:

Run actions/checkout@v2
Error: Invalid repository 'https://github.com/w3c/trace-context'. Expected format {owner}/{repo}.