Bump jekyll from 3.6.3 to 4.3.4 in /docs/_spec #32595

Job	Run time
test	52m 50s
test_non_bootstrapped	0s
test_scala2_library_tasty	0s
test_windows_fast	56m 34s
community_build_a	49m 42s
test_windows_full	0s
MiMa	10m 2s
community_build_b	1h 36m 36s
community_build_c	1h 3m 18s
test_sbt	17m 14s
build-msi-package	0s
test_java8	0s
build-sdk-package / build	8m 44s
build-chocolatey-package / build	18s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 55m 18s

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