Update experiment defaults and documentation to match GitHub Dependabot v0.296.0

[rhyskoedijk]

Update experiment defaults and documentation to match 0.296.0; This helps keep the DevOps Dependabot behavior aligned with what has been observed in GitHub.

Based on this GitHub Dependabot run:
/~https://github.com/tinglesoftware/dependabot-azure-devops/actions/runs/13232477372/job/36931750293

New experiments that are now enabled by default:

  'nuget-install-dotnet-sdks': true,
  'npm-fallback-version-above-v6': true,
  'npm-v6-deprecation-warning': true,
  'npm-v6-unsupported-error': true,
  'python-3-8-deprecation-warning': true,
  'python-3-8-unsupported-error': true,
  'enable-shared-helpers-command-timeout': true,
  'enable-fix-for-pnpm-no-change-error': true,
  'enable-engine-version-detection': true,
  'avoid-duplicate-updates-package-json': true,
  'allow-refresh-for-existing-pr-dependencies': true,

New experiments added to documentation:

Package Ecosystem	Experiment Name	Value Type	More Information
All	enable_shared_helpers_command_timeout	true/false	dependabot/dependabot-core#11125
All	allow_refresh_for_existing_pr_dependencies	true/false	dependabot/dependabot-core#11382
Bun	enable_bun_ecosystem	true/false	dependabot/dependabot-core#11446
NPM	npm_fallback_version_above_v6	true/false	dependabot/dependabot-core#10757
NPM	npm_v6_deprecation_warning	true/false	dependabot/dependabot-core#11112
NPM	npm_v6_unsupported_error	true/false	dependabot/dependabot-core#11112
NPM	enable_fix_for_pnpm_no_change_error	true/false	dependabot/dependabot-core#11291
NPM	enable_engine_version_detection	true/false	dependabot/dependabot-core#11392
NPM	avoid_duplicate_updates_package_json	true/false	dependabot/dependabot-core#11423
NuGet	nuget_install_dotnet_sdks	true/false	dependabot/dependabot-core#11090
Python	python_3_8_deprecation_warning	true/false	dependabot/dependabot-core#11166
Python	python_3_8_unsupported_error	true/false	dependabot/dependabot-core#11166