Reduce image size by rm'ing old tf versions

[nitrocode]

what

• Remove 5 old tf versions

why

• Reduce image size. Each binary is 80MB.
• This removes all terraform versions prior to 1.x.
• This will reduce the image size by 400MB.
• @jamengual and I worked on this and discussed this. Hoping this PR can be a discussion point with a wider audience.

references

• Example terraform version to download to check file size https://releases.hashicorp.com/terraform/0.12.31/
• Closes Reduce image size #2622
• This may also be required to merge feat: Download terraform versions using >= and ~> specifiers #1776

[chenrui333]

This is breaking change that we should call out separately.

[jamengual]

so we agreed to support only version 1.X to reduce the image size and document the change as a breaking change and announce it in Slack to the community

[chenrui333]

@nitrocode can we create an issue to track down this removal?

I can see there are some code cleanup opportunities as well

atlantis/server/core/runtime/plan_step_runner.go

Lines 117 to 122 in df94369

	// In version 0.9.* the workspace command was called env.
	workspaceCmd := "workspace"
	runningZeroPointNine := MustConstraint(">=0.9,<0.10").Check(tfVersion)
	if runningZeroPointNine {
	workspaceCmd = "env"
	}