Legacy Resolver does not work after pip 21.0

By default, the install.sh has the option to use the legacy resolver, "--use-deprecated=legacy-resolver." However, this won't work depending on the pip version installed on the user's host machine because this flag was removed in newer versions of pip (https://github.com/exasol/script-languages-release/issues/150). This could be addressed by documentation and by removing that flag from the default command.