update vm-instance module to support TPG v6

[RachaelSTamakloe]

This PR does the following:

• Removes the upper version constraint on vm-instance module to allow use of TPG v6
• reverts modifications made to guest accelerator in commit ca66b8c and defines guest_accelerator as a block

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[tpdownes]

Please modify the title/description of PR as discussed offline.

Took a second look and have concerns

[tpdownes]

Shouldn't you be reverting the change made to gpu_definition.tf in ca66b8c?

[tpdownes]

I performed a review tracing all use of guest_accelerator down to where the variable is really used. In most cases it traces to an instance template module. e.g.

https://github.com/terraform-google-modules/terraform-google-vm/blob/v10.1.1/modules/instance_template/main.tf

In each case, a dynamic guest_accelerator block is used in a TPG v6 forward-compatible way.