Improve release doc

[jpkrohling]

This PR improves the release docs by:

• removing outdated steps or information
• replacing ambiguous terms by more explicit references. For instance, origin and upstream were replaced by the GitHub references (namespace/repo-name)
• adding a table at the end of the document with the release history for the past few releases and the future schedule
• reordering some steps, like the ones about determining the version number and checking the state of the main branches

Signed-off-by: Juraci Paixão Kröhling [email protected]

[codecov]

Codecov Report

Merging #4881 (e2bf5e3) into main (2a2a722) will not change coverage.
The diff coverage is n/a.

❗ Current head e2bf5e3 differs from pull request most recent head 2051ce0. Consider uploading reports for the commit 2051ce0 to get more accurate results

@@           Coverage Diff           @@
##             main    #4881   +/-   ##
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  Coverage   90.64%   90.64%           
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  Files         180      180           
  Lines       10627    10627           
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  Hits         9633     9633           
  Misses        776      776           
  Partials      218      218           

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