[build] Use Spotless plugin to enforce/update copyright headers

[simonbasle]

This only applies to .java files changed compared to the origin base
branch (3.3.x), and excluding JDK stubs.

This allows to enforce license header with up-to-date end year only when
a file get touched (no need for a big bang update of the end year in
all files comes new year).

The year variable in the template is compatible with IntelliJ's own
template format (velocity), although IntelliJ doesn't typically include
the comment characters like in the added template file.

The template includes 2 blank lines at the end so that one blank line
remains after the license header once spotless is applied.

The ratchet (apply spotless only to files that changed between current
commit and the specified branch) is adapted depending on local build,
CI build without any specific parameter and CI build explicitly intended
for spotless (thanks to -PspotlessFrom=branch). The later is necessary
because spotless needs a full checkout, which is typically not done by
default by CI platforms like GitHub Actions.

See reactor/reactor#701

See also reactor/reactor#700 for the initial goal of including basic
formatting.

[simonbasle]

mmh actually, I'll revert the rename of the job so that there is continuity in the "required checks" of github actions

[reactorbot]

@simonbasle this PR seems to have been merged on a maintenance branch, please ensure the change is merge-forwarded to intermediate maintenance branches and up to main 🙇