Use `DjangoJSONEncoder` instead of custom `LazyStringEncoder` #15563

Job	Run time
test-sqlite (3.10, Django>=4.2,<4.3)	11m 8s
test-mysql (3.9, Django>=4.2,<4.3, false)	17m 30s
test-postgres (3.8, Django>=4.2,<4.3, false, --parallel)	7m 10s
test-postgres-elasticsearch7 (3.8, Django>=4.2,<4.3, false)	7m 18s
test-mysql (3.12, Django>=5.0,<5.1, false, --parallel, mysql:8.1)	13m 16s
test-postgres (3.11, Django>=5.0,<5.1, false, emailuser, --parallel)	7m 29s
test-sqlite-elasticsearch8 (3.10, Django>=5.0,<5.1, emailuser)	6m 11s
test-postgres (3.12, Django>=5.0,<5.1, postgres:15, notz, false, --parallel)	9m 55s
test-sqlite-opensearch2 (3.9, Django>=4.2,<4.3, false)	5m 19s
test-postgres (3.10, git+https://github.com/django/django.git@stable/5.0.x#egg=Django, true)	15m 40s
test-postgres (3.10, git+https://github.com/django/django.git@main#egg=Django, true, postgres:lat...	8m 13s
coverage	59s
	1h 50m 8s

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