Update header templates

[wiegandm]

What

This PR updates the header templates to use the SPDX-FileCopyrightText prefix for copyright information to aid automated processing of this information.

It also removes all "boilerplate" license text from the templates. The SPDX-License-Identifier is the authoritative statement for the license applicable for this file.

Additionally, issues with the templates for the GPL-3.0-or-later license for the XML and XSL formats were identified and fixed and a template for the GPL-2.0-only license for the NASL language was added.

Why

The changes are necessary to aid automated processing of license and copyright information.

[cfi-gb]

Closes #453 ?

[codecov]

Codecov Report

Merging #681 (1106de6) into main (e282290) will not change coverage.
The diff coverage is n/a.

❗ Current head 1106de6 differs from pull request most recent head e9692b9. Consider uploading reports for the commit e9692b9 to get more accurate results

@@           Coverage Diff           @@
##             main     #681   +/-   ##
=======================================
  Coverage   89.86%   89.86%           
=======================================
  Files          80       80           
  Lines        5229     5229           
  Branches      737      737           
=======================================
  Hits         4699     4699           
  Misses        373      373           
  Partials      157      157           

Impacted Files	Coverage Δ
pontos/updateheader/updateheader.py	84.35% <ø> (ø)

... and 1 file with indirect coverage changes

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