Bump minium required nodejs version to 14.x

The minimum required nodejs version for developing swh-web is 12.x but it reached EOL and is no longer maintained.

Also some of our javascript dependencies now require nodejs >= 14.x to be installed so it is time to upgrade it.

To do so we must:

upgrade nodejs in the swh/stack docker image used in our docker-compose environment
upgrade nodejs in the docker images used by Jenkins
bump nodejs version in package.json file of swh-web and update its readme
update developer documentation about the minimum required nodejs version

Migrated from T4259 (view on Phabricator)

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