Use 7.x as default Bazel version

Version 8 of Bazel introduces explicit requirement to select WORKSPACE
file as source of external dependencies. While it is quite
new there is still a need to support version 6.5 where new flag
cause build failure.

Signed-off-by: Anton Bondarenko <anton.bondarenko@arm.com>

Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Emil Ohlsson <emil.ohlsson@arm.com>