Change the license header to use SPDX abbreviations

In the proxy module and all other modules that are licensed under the GNU GPL, I'd like to switch from the full license header (as seen in HEADER.txt) to a modern SPDX declaration.

In every file, the header would then change to:

/**
 * SPDX-FileCopyrightText: Copyright (C) $YEAR Velocity Contributors
 * SPDX-License-Identifier: GPL-3.0-only
 */

For more context, see https://spdx.dev/learn/handling-license-info/

The api module is so small (and licensed differently), it might not even need it, but we may consider also using the SPDX declaration there.

This could also be a good oppertunity to bump the copyright year to 2018-2026 on all files, if a substantial change has been made to the file recently (this can probably be automated).

Start with HEADER.txt and inspect the proxy module plus other GPL-licensed modules to identify every affected file. Confirm whether the differently licensed api module is included, then verify that all applicable files use the SPDX-FileCopyrightText and SPDX-License-Identifier declarations and that any copyright-year updates are justified.