Remove years from existing license headers

[friegger]

Summary

Update the license headers in all existing tracked code files, REUSE.toml and other files to use the year-free SPDX copyright text.

Reference implementation: ironcore-dev/metal-operator#1160

Rationale

PR ironcore-dev/metal-operator#1160 updates hack/license-header.txt, but addlicense does not rewrite files that already contain an SPDX license marker.
Existing Go files can therefore retain year-bearing copyright headers.

Affected areas

• All tracked code files with existing license headers.
• License automation, if a dedicated rewrite step is required.
• hack/license-header.txt and the Makefile targets that manage license headers.

Required changes

• Replace year-bearing SPDX-FileCopyrightText headers in existing tracked Go files with the current year-free header.
• Preserve each code file's SPDX license marker.
• Add or document a safe rewrite process if the current tooling cannot update existing headers.

Acceptance criteria

• No tracked code file has a year in its license header.
• All updated code files retain a valid SPDX license marker.
• make check-license continues to pass after the header migration.