Fix validate workflow: update action versions and case-insensitive readme check

[Copilot]

The validation workflow was failing due to a case-sensitive readme check and outdated action versions.

Changes

• Case-insensitive readme check: Replaced exact readme.md filename match with find -type f -iname so README.md, Readme.md, etc. all pass, while correctly requiring a regular file (not a directory)
• Action versions bumped to latest major:
  • actions/checkout: v4 → v6
  • actions/setup-python: v5 → v6
  • actions/setup-node: v4 → v6
  • actions/setup-java: v4 → v5
  • actions/setup-dotnet: v4 → v5
  • actions/setup-go: v5 → v6
• .NET version: Updated from 9.0 to 10.0

# Before
if [ ! -f "${sample}README.md" ]; then

# After — file-only, case-insensitive, stops at first match
if ! find "${sample}" -maxdepth 1 -type f -iname "readme.md" -print -quit | grep -q .; then

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[Copilot]

Pull request overview

This PR updates the repository’s GitHub Actions validation workflow to be more tolerant of README filename casing and to modernize referenced GitHub Action versions.

Changes:

• Make the sample README presence check case-insensitive using find -iname.
• Bump several actions/* dependencies to newer major tags across all validation jobs.

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