Custom licenses are harder to use - standardization is key

We can simplify our license headers in the files to a single line

Practically there are no changes, just cleanups: PMD is and stays open source under the BSD license.

These are the changes:

• Use "PMD Development Team" as copyright owner instead of InfoEther. InfoEther is no longer involved in actively developing PMD - and PMD is open source anyway.
• Remove the "advertising clause" that requests to mention
  "This product includes software developed in part by support from the Defense Advanced
  Research Project Agency (DARPA)" in the end user documentation. That's long history. And apparently, it is not done. This sentence is only found in the license text, but not in the documentation.
• Adjust file headers to use SPDX identifier, see https://spdx.org/licenses/ and https://spdx.dev/ids/
  Example: // SPDX-License-Identifier: BSD-3-Clause
• Possible Licenses: https://spdx.org/licenses/BSD-3-Clause.html or https://spdx.org/licenses/BSD-2-Clause.html
  • the BSD-3-Clause license is the closest match. It contains the 3rd clause "Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission." - which means that PMD or our names must no be used without our consent.
• See also Who is InfoEther, LLC in License and what does it mean for usage? #3050
• See also https://opensource.stackexchange.com/questions/11556/can-i-use-apache-2-0-licensed-project-inside-my-bsd-3-clause-licensed-project