fix: pin pillow and dotnet Lambda.Tools to fix CI failures

[ckawl]

• Pin pillow<=12.2.0 for python3.10 and python3.11 ML templates (scikit, pytorch, xgboost). pillow 12.3.0 has no manylinux2014 wheel, requiring gcc compilation which is not available in Lambda base images.
• Pin Amazon.Lambda.Tools to 5.14.0 for dotnet6 image template. Version 7.0.0 dropped .NET 6 support.

Issue #, if available:
N/A — pre-existing CI failures

Description of changes:

Fix CI build failures in python3.10/3.11 ML templates and dotnet6 image template:

• pillow<=12.2.0: pillow 12.3.0 does not ship a manylinux2014 wheel, causing pip to download the source tarball and attempt compilation with gcc (which is not available in Lambda base images).
  Pinning to <=12.2.0 ensures a pre-built wheel is used.
• Amazon.Lambda.Tools --version 5.14.0: Version 7.0.0 dropped .NET 6.0 support, breaking the dotnet6 image build. Pinning to the last compatible version.

Affected templates:

• python3.10: apigw-scikit, apigw-pytorch, apigw-xgboost
• python3.11: apigw-scikit, apigw-pytorch, apigw-xgboost
• dotnet6: hello-img

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Description of changes:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.