51Degrees RTD submodule: small improvements and fixes

[jwrosewell]

Type of change

• Updated RTD submodule

Description of change

This pull request introduces several improvements and fixes to the 51Degrees RTD submodule in Prebid.js.

• Added support for screenpixelsphysicalheight and screenpixelsphysicalwidth:
  • These properties are now used instead of screenpixelsheight and screenpixelswidth when available.
  • Enables more accurate PPI calculation using physical screen dimensions.
• Performed minor refactoring to prepare for upcoming enhancements that will include additional data points in the 51Degrees RTD submodule.
• Eased module configuration:
  • Addressed an issue where some third-party Prebid.js wrappers force users to input both resourceKey and onPremiseJSUrl, leading to invalid configurations.
  • Users can now set the redundant parameter to the string "0", which the module will ignore, allowing for a valid configuration.
• Documentation updates:
  • Replaced an outdated template URL with the current one.
  • Included information about using link preconnect for optimisation purposes.
  • Included guidance in the documentation for users facing configuration issues.
• Included the 51Degrees RTD submodule in .submodules.json, which was previously omitted.
• Added new tests. Code coverage is still at 100%.