Trim license boilerplate to the SPDX identifier

[davidharrishmc]

The full Solderpad/Apache license text lives in LICENSE. This removes the repeated license paragraph from 372 file headers and keeps the copyright line and SPDX-License-Identifier: Apache-2.0 WITH SHL-2.1. Comment-only change; no code is modified.

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[davidharrishmc]

Review notes: the change is comment-only (3868 deletions, no insertions, every file keeps its SPDX line), but:

• Nine files added to main since this branched still carry the full license paragraph: tests/coverage/{cbozwriteback,divflushstall,icacheITLBUpdate,noncanonical,svadumxr,svinvalfence,tlbADdup,tlbMultiMatch}.S and tests/coverage32/divflushstall32.S. Merge main and rerun the trim.
• Clean up src/ file headers #1954 contains this PR's commit, so either merge this first or close it in favor of Clean up src/ file headers #1954.
• It conflicts with the Clean up comments and whitespace in src/; delete unused modules #1952 stack (modify/delete on the four modules Clean up comments and whitespace in src/; delete unused modules #1952 deletes: arrs.sv, aesmixcolumns8.sv, rotate.sv, trickbox_apb.sv), so the merge order matters.
• Since this changes the license notice project-wide, an explicit ack from a co-maintainer or OpenHW would be prudent.

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[davidharrishmc]

Merge order: #1954 contains this PR's commit. Merge this first and then #1954, or close this one in favour of #1954. Either way, both should land after the bug-fix PRs and before the #1952 → #1955 → #1959 stack, or be resolved against it: the stack deletes four files this PR edits (arrs.sv, aesmixcolumns8.sv, rotate.sv, trickbox_apb.sv).

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[davidharrishmc]

Updated per review: merged main and trimmed the license paragraph from the 20 coverage tests added on main since this branch was made (the 9 noted in the review plus 11 that landed after it, including the wfi tests from #1941 and fpADUpdate from #1931). No file outside LICENSE still carries the paragraph. The change is still comment-only: 372 files, deletions only.

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