Relicense KeY from GPLv2-only to GPLv2-or-later

[wadoon]

By AI in 140 kTokens. The prompt:

The license is currently gplv2-only. Please change the license, and the license header to gplv2-or-later. The license header is set by spotless via Gradle. Please check if there are other signs of gpl2-only, .e.g, in POM generation or license dialog.

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Change the KeY license from "GPL-2.0-only" to "GPL-2.0-or-later" (SPDX GPL-2.0-or-later), i.e. "GNU General Public License Version 2, or (at your option) any later version".

License headers (set by spotless via Gradle):

• Update the spotless header template gradle/header to the new wording and run spotlessApply to rewrite the headers of all Java files in the main build (~2770 files).
• Update the headers of the taclet rule files (*.key, where the spotless Key-format header step is currently disabled) and of the standalone scripts/tools/mt project.
• Update the header template embedded in the source generators GenerateUnitTests/GenerateUnitTestsUtil so generated tests carry the new notice, and fix the two files spotless did not cover (package-info.java, fully commented-out AssertCommand.java).

Maven POM generation:

• Declare "GNU General Public License (GPL), Version 2, or later" in the mavenPublishing licenses block used to generate the per-module POMs.

License dialog:

• Update the KEY_FALLBACK text of LicenseAction to "GNU General Public License v2, or (at your option) any later version".

Documentation and license texts:

• LICENSE.TXT: the declaration now reads "either version 2 of the License, or (at your option) any later version".
• README.md, CONTRIBUTING.md and .github/pull_request_template.md updated; removed the "(no later versions)" and "GPLv2 (only)" wording.

Deliberately left unchanged:

• key.ui/examples/case-studies/timsort/.../TimSort.java: verbatim third-party OpenJDK copy (Oracle copyright) used as the case-study subject.
• scripts/mvndeploy/*.pom.xml: POMs describing third-party libraries (recoder, docking-frames).

Validation: spotlessCheck passes; no GPL-2.0-only references remain in tracked files.