Replace full license text in source files by SPDX reference

[dennisvang]

Here's from https://spdx.dev/learn/handling-license-info/:

Why use SPDX IDs?

• Determine easily which licenses apply to a file
• Standardize licenses in source code, docs and others
• Eliminate error-prone parsing of license headers
• Decrease confusion by using the SPDX License List

Read more about why to use SPDX identifiers
How do SPDX IDs work?

In each file in your project, just add a single line in the following format, tailored to your license(s) and the comment style for that file’s language:

• // SPDX-License-Identifier: MIT
• /* SPDX-License-Identifier: MIT OR Apache-2.0 */
• # SPDX-License-Identifier: GPL-2.0-or-later

Read more about how to use SPDX identifiers
Where are SPDX IDs used?

SPDX identifiers are being used in a growing number of open source projects (such as the Linux kernel), licensing guidelines (such as the FSFE‘s REUSE Software initiative), and license scanning tools.