ci: remove minimum coverage setting

[Jusshersmith]

Problem

With the recent introduction of codecov, builds are failing to pass checks if their coverage falls below that of master.

Solution

• Although we want to increase coverage, I'm not sure if we want to set those hard limitations immediately?

  If I understand correctly these changes should remove the 'target', so we'll still get the PR comment but it won't fail due to coverage levels.

• This also adds the codecov badge to our readme.

[codecov]

Codecov Report

Merging #214 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
- Coverage   61.23%   61.21%   -0.02%     
==========================================
  Files          50       50              
  Lines        4073     4071       -2     
==========================================
- Hits         2494     2492       -2     
  Misses       1393     1393              
  Partials      186      186

Impacted Files	Coverage Δ
internal/auth/providers/okta.go	61.57% <0%> (-0.32%)	⬇️

[Jusshersmith]

Would we prefer to set a threshold perhaps? (allow the PR to drop by x% (5%?), but still pass)

[jphines]

@Jusshersmith the threshold SGTM, maybe 2-3%?

[Jusshersmith]

🙏 @jphines, please review/re-approve when you have the chance. Updated comments slightly to give some information around validating config.