Add https://stackoverflow.com to ignore in linkcheck, and sort entries #13090

Job	Run time
Core Basic (18.x)	7m 5s
Core Basic (20.x)	6m 50s
Core Blocks (18.x)	7m 9s
Core Control Panels (18.x)	4m 14s
Core Content (18.x)	3m 5s
Core Blocks (20.x)	6m 53s
Core Control Panels (20.x)	3m 51s
Core Content (20.x)	3m 43s
Core Basic - Plone 5 (18.x)	7m 37s
Core Blocks - Listing (18.x)	5m 20s
Coresandbox (18.x)	3m 33s
Core Volto Slate (18.x)	7m 7s
Core Blocks - Listing (20.x)	5m 43s
Multilingual (18.x)	2m 37s
Core Volto Slate (20.x)	6m 48s
Working Copy (18.x)	3m 27s
Project Generator (18.x)	5m 18s
Multilingual in Seamless Mode (18.x)	2m 56s
Project Generator (20.x)	5m 9s
Seamless Mode (18.x)	7m 7s
	1h 45m 32s

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