[ci] update versions of ES, GitHub Actions

[jameslamb]

It's been about 7 months since the last commit in this project (#217). This PR updates a few versions of things in CI as standard maintenance.

• Elasticsearch versions (based on https://www.elastic.co/downloads/past-releases)
• versions of all GitHub Actions used
• version of R (4.0.3 --> 4.1.0)

[jameslamb]

https://github.com/actions/checkout uses v2 the way some projects use a stable tag. For example, if they publish a 2.3.5, at that moment v2.3.5 and v2 will point to the same commit.

So this is equivalent to doing something like >=2.0.0,<3.0.0 in Python

[jameslamb]

R jobs are failing with the following

Error: --- re-building ‘FAQ.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) not available. Falling back to R Markdown v1.
Error: Error: processing vignette 'FAQ.Rmd' failed with diagnostics:
The 'markdown' package should be installed and declared as a dependency of the 'uptasticsearch' package (e.g., in the 'Suggests' field of DESCRIPTION), because the latter contains vignette(s) built with the 'markdown' package. Please see https://github.com/yihui/knitr/issues/1864 for more information.
--- failed re-building ‘FAQ.Rmd’

SUMMARY: processing the following file failed:
  ‘FAQ.Rmd’

Error: Error: Vignette re-building failed.
Execution halted
Error: Process completed with exit code 1.

According to https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-README.md, pandoc isn't guaranteed to be available. I'll add it here.

[jameslamb]

installing packages is taking 8+ minutes (!!!), so I'm taking the opportunity to also add Ncpus = parallel::detectCores() here in install.packages().

[codecov-commenter]

Codecov Report

Merging #220 (2e4e3a3) into master (dd59ddd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   95.47%   95.47%           
=======================================
  Files           8        8           
  Lines         597      597           
=======================================
  Hits          570      570           
  Misses         27       27           

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