fix: drop node12.x from publish script

[astuyve]

What does this PR do?

Unfortunately this caused an off by one error where we'd apply the wrong compatible-runtime flag to published layers.

Fortunately this flag is not actually enforced in Lambda, and is a label only. So this has no runtime implications.

Searching the publish_layers codebase, we see this is only used for setting this flag.

Motivation

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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