Update SPDX headers in source files

[bact]

• Put copyright texts to SPDX-FileCopyrightText:
  • For example: Copyright (c) 2023 Source Auditor Inc. -> SPDX-FileCopyrightText: Copyright (c) 2023 Source Auditor Inc.
• Add SPDX-FileType: SOURCE
• Follow header pattern since 2024 that do not include the content of the license
• These should allow better parsing of copyright information and file type, also reduce the size of source files a bit
• Add missing copyright header - using this template Copyright (c) YYYY Source Auditor Inc. where YYYY is the first year that that file appears on the GitHub repo
• Add few missing Javadoc
• See also: Add SPDX-FileCopyrightText and SPDX-FileType to source file header Spdx-Java-Library#291

[goneall]

LGTM