fix(cli): remove legacy stack tags behavior

[rix0rrr]

Here's some nasty legacy around Stack Tags:

• Before CDK v1.33.0, we wrote stack tags to the construct metadata, and they were written as a { Key, Value } object that got directly passed into the CloudFormation CreateChangeSet API.
• In CDK v1.33.0 (nearly 6 years ago), we formalized the cloud assembly schema into a jsii package, so we had to type the tag object as { key, value } in memory (otherwise jsii would yell at us). For backwards compatibility with the on-disk format and the (extracted) JSONSchema Validation, we used to re-case the identifiers: just before they were written to disk we would change key → Key and just after reading from disk but before validation and we would change Key → key.
• In CDK v1.66.0 (5 and a half years ago), we added a tags property to the CloudFormation Stack artifact in the CDK manifest, and the CLI and other tools stopped using the stack metadata to read stack tags.

Since this is to support end-of-life CDKs, in this PR I'm taking some of that complexity out; we stop converting key → Key when writing a cloud assembly manifest -- but we do still have to convert Key → key in order to pass JSONSchema validation as we read the manifest. Tags are exclusively read from the artifact properties, there will be no fallback to the metadata anymore.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.69%. Comparing base (937452b) to head (7ee1cd0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1053      +/-   ##
==========================================
- Coverage   87.70%   87.69%   -0.01%     
==========================================
  Files          72       72              
  Lines       10095    10095              
  Branches     1332     1332              
==========================================
- Hits         8854     8853       -1     
- Misses       1216     1217       +1     
  Partials       25       25              

Flag	Coverage Δ
suite.unit	87.69% <ø> (-0.01%)	⬇️

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