Remove legacy shell quoting and FF_USE_NEW_SHELL_ESCAPE feature flag

What does this MR do?

It removes legacy shell quoting in favour of the new shell escape. As the FF_USE_NEW_SHELL_ESCAPE is made the default behaviour, the FF_USE_NEW_SHELL_ESCAPE has also been removed

Why was this MR needed?

The legacy shell quoting implementation was problematic and is greatly improved by the new implementation. The new shell quoting was already used for variable expansion and with this MR it is just expanded to the whole ci script.

No changes in behaviour is expected with the new implementation.

What's the best way to test this MR?

Pipeline passes ✅

What are the relevant issue numbers?

close #29368 (closed)