Support local backend

[Nuru]

what

• Support retrieving remote state from local backends
  • NOTE: Using relative paths in local backends is tricky, because the path needs to resolve to the same directory from the source root module directory as from the client root module directory.
• Fix Terratests
  • The spacelift test suite is broken, and we never previously required it to work, so it is now skipped
• Update test suite to use go v1.21 and update dependencies

why

• When running demos and tests, it is more convenient to use local backends
• Maintain some degree of testing
• Closes multiple dependabot PRs:
  • Supersedes and Closes Bump google.golang.org/protobuf from 1.28.1 to 1.33.0 in /test/src #82
  • Supersedes and Closes Bump golang.org/x/crypto from 0.1.0 to 0.17.0 in /test/src #83
  • Supersedes and Closes Bump google.golang.org/grpc from 1.51.0 to 1.56.3 in /test/src #84
  • Supersedes and Closes Bump golang.org/x/net from 0.8.0 to 0.23.0 in /test/src #87
  • Supersedes and Closes Bump github.com/hashicorp/go-getter from 1.7.1 to 1.7.4 in /test/src #88

[mergify]

Important

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[Nuru]

/terratest

[github-actions]

These changes were released in v1.7.0.