fix(module/lb_external): Setting balancing_mode = CONNECTION due to provider changes

[pavelrn]

Description

Set google_compute_region_backend_service balancing_mode = "CONNECTION" explicitly due to the change in the terraform-provider-google version 6.0+

Motivation and Context

Resolves #34

How Has This Been Tested?

Create a load balancer as part of Common architecture.

Screenshots (if appropriate)

n/a

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes if appropriate.
• [ ] All new and existing tests passed.

[lstadnik]

It was addressed in PR #39