[MRG] fix links, remove ubuntu 20.04 from CI

[sappelhoff]

PR Description

I suspect CI will also break for the Ubuntu 20.04 workflows, as it does in our scheduled workflow. We'll need to adjust it somehow, maybe by cutting it out -- it doesn't seem to be specifically needed. I'll look into it later.

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
  - [ ] whats_new.rst is updated
  - [ ] New contributors have been added to CITATION.cff
  - [ ] PR description includes phrase "closes <#issue-number>"

[agramfort]

do not know why CIs fails

[larsoner]

+1 for just cutting 20.04 out of the build matrix

[codecov]

Codecov Report

Merging #1111 (d15a96e) into main (79439c8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1111   +/-   ##
=======================================
  Coverage   95.27%   95.27%           
=======================================
  Files          26       26           
  Lines        3912     3912           
=======================================
  Hits         3727     3727           
  Misses        185      185           

Impacted Files	Coverage Δ
mne_bids/dig.py	98.24% <ø> (ø)
mne_bids/path.py	97.70% <ø> (ø)

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