Merge bootstrap4 sxs support from release/6.0

[HaoK]

Replacing #36397

[HaoK]

@dougbu do you have any ideas on what I can try to resolve this weird error with this backport of a change from release/6.0? I can't reproduce this build error locally at all, I did a full git clean/nuke of artifacts, and even tried running .\build.cmd -ci but things build fine locally for me.

[dougbu]

@dougbu do you have any ideas on what I can try to resolve this weird error with this backport of a change from release/6.0?

It appears the <GenerateStaticWebAsssetsPropsFile /> task in the 7.0.100-alpha.1.21474.3 SDK currently used for the 'main' branch is not compatible with the options you're using. We moved to the 6.0.100-rtm.21477.15 SDK in the 'release/6.0' branch just one day after the most recent SDK change in 'main'. So, this is indeed a bit strange.

Thoughts @javiercn❔ You made the last few changes to this task. Does @HaoK just need to wait for #37192 to bring us to the 7.0.100-alpha.1.21480.20 SDK❔

[HaoK]

Yeah I don't understand why I can't reproduce this locally either...

[HaoK]

Replaced by #38865