meson: fix SPDX identifiers
LGPL-2.1+ identifier was deprecated as of SPDX version 2.0rc2, hence use the current version: LGPL-2.1-or-later.
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LGPL-2.1+ identifier was deprecated as of SPDX version 2.0rc2, hence use the current version: LGPL-2.1-or-later.