Update yarn install command for 4.3.x line

[tabletcorry]

The current yarn install documentation states to use --pure-lockfile which appears to be completely invalid in Yarn 4, and will fail in the 4.3.x code line.

# When run against Yarn 4
+ yarn install --pure-lockfile
Unknown Syntax Error: Unsupported option name ("--pure-lockfile").

The command should be updated to yarn install --immutable as discussed in discord.

CC @ThisIsMissEm

[vmstan]

Added enhancement tag to indicate need to hold for 4.3 release.