DO NOT REVIEW: Update BASE_TAG default version to 6.8.0 #35575

Job	Run time
Determine Matrix	4s
Multisite Default / Test before	14m 6s
Multisite Default / Test current	13m 58s
Multisite Memcached / Test before	15m 1s
Multisite Memcached / Test current	14m 16s
Single Site Default / Test before	6h 0m 6s
Single Site Default / Test current	13m 48s
Single Site Memcached / Test current	13m 58s
Single Site Memcached / Test before	14m 55s
Slack Notifications	0s
matrix.label	0s
Failed workflow tasks	-1s
	7h 40m 11s

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