[GcpNfsVolume] Remove support for Legacy tiers

[dushanpantic]

Description

The GcpNfsVolume API should not offer support for legacy tiers.
There are alternatives offered by Google that replace the legacy tiers, and we are already supporting them.
Remove support for STANDARD, PREMIUM, ENTERPRISE, HIGH_SCALE_SSD tier.

Our documentation already (prior to this change) does not list legacy tiers as supported. However, our reconciler will allow creation of GcpNfsInstance with legacy tier, but it will go into Error state immediately.
On the other hand, our UI still lists some of the legacy tiers:

Prerequisites for removal:

• No instance of legacy tier exists in any environment
  • dev
  • stage
  • prod

Reasons

We don't want to support legacy tiers, when non-legacy with feature parity is already offered by Google.

Attachments