[doc] Update copyright year #2021

Job	Run time
build (pypy3.7, wikipedia:en, true)	21m 23s
build (pypy3.7, wikisource:zh, true)	21m 24s
build (3.7, wikipedia:en, true)	21m 26s
build (3.7, wikisource:zh, true)	17m 36s
build (3.8, wikipedia:en, true)	21m 24s
build (3.8, wikisource:zh, true)	21m 24s
build (3.9, wikipedia:en, true)	21m 24s
build (3.9, wikisource:zh, true)	20m 21s
build (3.10, wikipedia:en, true)	21m 26s
build (3.10, wikisource:zh, true)	21m 27s
build (3.11, wikipedia:en, true)	21m 25s
build (3.11, wikisource:zh, true)	21m 26s
build (3.7, wikipedia:test)	21m 24s
build (3.7, false, wpbeta, en)	21m 27s
build (3.7, false, wpbeta, zh)	19m 8s
build (3.8, wsbeta:en)	18m 23s
build (3.7, wikidata:test)	19m 30s
build (3.9, wiktionary:ar, true)	3m 42s
build (3.7, wikidata:wikidata)	2m 55s
build (3.8, wowwiki:uk)	2m 11s
build (3, wikipedia:de, macOS-latest)	1m 47s
build (3.12, wikipedia:en, true, true)	55s
build (3.12, wikisource:zh, true, true)	1s
build (3.13-dev, wikipedia:test, true)	1s
	6h 3m 30s

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