Upgrade codecov action to v2

[rossviljoen]

Currently getting this issue:
https://community.codecov.com/t/codecov-github-action-fails-could-not-find-a-repository-try-using-upload-token/3081

[st--]

Still not working unfortunately, though now with a different error message:

[2021-09-30T12:35:03.165Z] ['info'] Pinging Codecov: https://codecov.io/upload/v4?package=github-action-2.1.0-uploader-0.1.2&token=*******&branch=ross%2Fcodecov-fix&build=1291013778&build_url=https%3A%2F%2Fgithub.com%2FJuliaGaussianProcesses%2FApproximateGPs.jl%2Factions%2Fruns%2F1291013778&commit=bc4424a09cd3dd7f81bdcbd3cc55668a3e305474&job=CI&pr=63&service=github-actions&slug=JuliaGaussianProcesses%2FApproximateGPs.jl&name=&tag=&flags=&parent=
[2021-09-30T12:35:03.477Z] ['error'] Error POSTing to https://codecov.io: 404 {'detail': ErrorDetail(string='Could not find a repository, try using repo upload token', code='not_found')}
[2021-09-30T12:35:03.477Z] ['error'] There was an error running the uploader: Error uploading to https://codecov.io: Error: Not Found

[st--]

This does not solve the underlying issue - @rossviljoen just found out that this is due to the repo rename, see https://codecov.freshdesk.com/support/solutions/articles/43000629044-repo-not-updating-after-transfer-rename - so will have to be sorted out separately. Regardless, upgrading to v2 is the right thing to do (v1 is being sunset), so let's merge it in.

[codecov]

Codecov Report

Merging #63 (bc4424a) into master (7c5c5c7) will decrease coverage by 1.78%.
The diff coverage is 86.81%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   88.77%   86.99%   -1.79%     
==========================================
  Files           3        5       +2     
  Lines          98      246     +148     
==========================================
+ Hits           87      214     +127     
- Misses         11       32      +21     

Impacted Files	Coverage Δ
src/expected_loglik.jl	86.11% <86.11%> (ø)
src/laplace.jl	86.89% <86.89%> (ø)
src/elbo.jl	82.35% <100.00%> (-5.65%)	⬇️

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[rossviljoen]

The rename issue is now fixed as well