refactor: Mark semantically final variables as final (or const) proper [DCM]

[luanpotter]

Description

I was playing around with the rule avoid-global-state
While I don't think we should enable it, because we do have several totally legitimate cases of what the rule considers global state, it did help me find any current cases where the things just should be final constants.
So this PR will mark semantically final variables as final (or const) proper, exclusively on examples (no violations on actual src code are legit).

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.