Add SPDX header files

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

gradle/wrapper/gradle-wrapper.properties.license

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+SPDX-FileCopyrightText: 2017-2024 Nextcloud GmbH and Nextcloud contributors
+SPDX-License-Identifier: GPL-3.0-or-later

gradlew.bat.license

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+SPDX-FileCopyrightText: 2015 the original author or authors.
+SPDX-License-Identifier: Apache-2.0

gradlew.license

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+SPDX-FileCopyrightText: 2015-2021 the original author or authors.
+SPDX-License-Identifier: Apache-2.0