Update web.config max upload size defaults, correct backend calcs, and fix validation issue preventing increasing

[david-poindexter]

Summary

Resolves #5778

The following was done in this PR:

• Updated default values for maxRequestLength in development.config and release.config.
• Added section for IIS7 support of maxAllowedContentLength

<system.webServer>
    <security>
      <requestFiltering>
        <requestLimits maxAllowedContentLength="28672000" />
      </requestFiltering>
    </security>
</system.webServer>

• Resolved backend calculation issues resulting in confusing values (by default and when saving)
• Resolved validation issue that was preventing an admin from increasing the Max Upload Size in the UI. Now it validates against the max supported value of 4GB (minus one byte) for IIS.

Lastly, we decided it best to not add a new field in the UI to control requestLengthDiskThreshold. Since this is not something DNN admins would typically know/understand, it is best to leave this to manual web.config changes by server admins wanting to optimize performance for specific scenarios/environments.

[valadas]

Awesome, just one value is not the "Microsoft default" and is quite big here

[bdukes]

👍🏻