Fix rxjs operators import to support v.6.x - master

[ddaribo]

Closes #14654

Additional information (check all that apply):

• [x] Bug fix
• [ ] New functionality
• [ ] Documentation
• [ ] Demos
• [ ] CI/CD

Checklist:

• [x] All relevant tags have been applied to this PR
• [ ] This PR includes unit tests covering all the new code (test guidelines)
• [ ] This PR includes API docs for newly added methods/properties (api docs guidelines)
• [ ] This PR includes feature/README.MD updates for the feature docs
• [ ] This PR includes general feature table updates in the root README.MD
• [ ] This PR includes CHANGELOG.MD updates for newly added functionality
• [ ] This PR contains breaking changes
• [ ] This PR includes ng update migrations for the breaking changes (migrations guidelines)
• [ ] This PR includes behavioral changes and the feature specification has been updated with them

[damyanpetev]

This fix is good, considering ./operators is an active export path still, even though it is considered deprecated after 7.2 https://rxjs.dev/guide/importing#new-in-rxjs-v720

Unfortunately, if we're to adhere to 6 support alongside Angular there's currently nothing preventing new code from going with the latest as documented. Haven't gone though the entire changelog since 6 is really quite old at this point, so somewhat hesitant to suggest downgrading the entire monorepo to 6. If possible only for the "bundle-test" project or perhaps an special CI step to downgrade and re-build to guarantee no other imports like that creep in.

Also noticed we don't really list rxjs explicitly in the lib peerDependencies and wondering if that's even right. If we do, we could list 7+ at some point too.