Use CentOS Stream 8 instead of CentOS Linux 8

[ekohl]

CentOS Linux 8 went End Of Life at the end of 2021 and at the end of January it will disappear from the mirrors. Stream is the supported replacement. By using the Vagrant box and container image it should mean most testing will not notice the difference.

[codecov]

Codecov Report

Merging #240 (443729f) into master (605320e) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head 443729f differs from pull request most recent head a4bdd3b. Consider uploading reports for the commit a4bdd3b to get more accurate results

@@            Coverage Diff            @@
##           master    #240      +/-   ##
=========================================
- Coverage    0.52%   0.52%   -0.01%     
=========================================
  Files          14      14              
  Lines        2473    2477       +4     
=========================================
  Hits           13      13              
- Misses       2460    2464       +4     

Impacted Files	Coverage Δ
lib/beaker-hostgenerator/data.rb	0.00% <0.00%> (ø)

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