Update OS images, AWS SDK dependencies, and README heading

[ydesgagn]

Summary

This pull request updates the operating system images used in CI and the Docker base image to newer releases, refreshes the AWS SDK gem dependencies, and corrects a heading level in the README.

Key changes:

• Update GitHub Actions macos-latest runners to macos-26 in build workflow matrices
• Update Dockerfile base image from ubuntu:24.04 to ubuntu:26.04
• Bump AWS SDK gems (autoscaling, cloudformation, cloudfront, cloudwatchlogs, core, ec2, elasticloadbalancingv2, iam, kms, lambda) and aws-partitions to their latest versions in Gemfile.lock
• Fix Windows Installation heading level in README.md (h5 → h4) for consistent document structure
• Refresh .soup.json and docs/soup.md to match updated dependency versions

Types of changes

• Bugfix (fixes an issue)
• New feature (adds functionality)
• Refactoring (improves code without changing functionality)
• Breaking change (incompatible changes)
• Build or security update (updates dependencies, libraries, or security patches)
• Code style or documentation update (formatting, renaming, or documentation changes)
• Other (please describe):

Checklist

• Unit tests added to validate my fix/feature
• I have manually tested my change
• I did not add automation test. Why ?:
• Database changes requiring migration with downtime or reprocessing of existing data
• The SOUP file lists the risk Level, requirements and verification reasoning associated with each library
• readme.md includes sections on introduction, installation, usage, and contributing
• docs/architecture.md includes sections on the architecture diagram, software units, software of unknown provenance, critical algorithms and risk controls related to PII and security
• Impact on PII, privacy regulations (CCPA/GDPR/PIPEDA), CIS benchmarks or security (availability/confidentiality/integrity); management must be notified