fix pin-and-downgrade

[ghost]

Previously, if you tried to pin to a lower version, we would treat it as a no-op (and keep you at your current version). Now, we'll perform the update ("downdate").

This resolves #187.

We've had a few back-and-forths in this code, most recently #137. I added unit tests to try and put this to bed for good, or at least prevent further regressions.

Standard checks

• Unit tests: I added some!
• Docs: none needed
• Backwards compatibility: no issues

[github-actions]

Package	Line Rate	Health
github.com/klothoplatform/klotho/pkg/analytics	3%	❌
github.com/klothoplatform/klotho/pkg/annotation	23%	❌
github.com/klothoplatform/klotho/pkg/cli	4%	❌
github.com/klothoplatform/klotho/pkg/core	21%	❌
github.com/klothoplatform/klotho/pkg/env_var	82%	✔
github.com/klothoplatform/klotho/pkg/exec_unit	54%	➖
github.com/klothoplatform/klotho/pkg/infra/kubernetes	59%	➖
github.com/klothoplatform/klotho/pkg/infra/kubernetes/helm	39%	❌
github.com/klothoplatform/klotho/pkg/input	63%	➖
github.com/klothoplatform/klotho/pkg/lang	38%	❌
github.com/klothoplatform/klotho/pkg/lang/dockerfile	0%	❌
github.com/klothoplatform/klotho/pkg/lang/golang	33%	❌
github.com/klothoplatform/klotho/pkg/lang/javascript	48%	❌
github.com/klothoplatform/klotho/pkg/lang/python	61%	➖
github.com/klothoplatform/klotho/pkg/lang/yaml	0%	❌
github.com/klothoplatform/klotho/pkg/logging	6%	❌
github.com/klothoplatform/klotho/pkg/multierr	95%	✔
github.com/klothoplatform/klotho/pkg/provider/aws	60%	➖
github.com/klothoplatform/klotho/pkg/runtime	78%	✔
github.com/klothoplatform/klotho/pkg/static_unit	33%	❌
github.com/klothoplatform/klotho/pkg/updater	31%	❌
github.com/klothoplatform/klotho/pkg/validation	73%	➖
github.com/klothoplatform/klotho/pkg/yaml_util	79%	✔
Summary	41% (4000 / 9648)	❌