fixes for case adding data at the end of the textbox (#3338)

## Fixes #3335

When using the TextBoxMask extension, the TextBox text not updated if the text with the same string length changes, unless if the string length is less than the current Text length.

## PR Type
What kind of change does this PR introduce?
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- Bugfix
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
<!-- - Sample app changes -->
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## What is the current behavior?
Example, a TextBox with Mask `99:59:59` where cutom mask 5:[0-5], if the current Text is 10:20:30 and we update with 20:30:40, the Text did not changed/updated. The UI will show the same '10:20:30' because of this code: `textbox.Text = oldText;`

## What is the new behavior?
Example, if the current Text is 10:20:30 and we update with 20:30:40, the Text changed/updated. The fix is to let the text updated on this regards and no other behavior changes.


## PR Checklist

Please check if your PR fulfills the following requirements:

- [ ] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->
- [ ] Sample in sample app has been added / updated (for bug fixes / features)
    - [ ] Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)
- [ ] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [ ] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

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## Other information

Author: msftbot[bot]

Microsoft.Toolkit.Uwp.UI/Extensions/TextBoxMask/TextBoxMask.cs

@@ -300,7 +300,7 @@ private static void Textbox_TextChanging(TextBox textbox, TextBoxTextChangingEve
             // case adding data at the end of the textbox
             if (oldSelectionStart >= oldText.Length && !isDeleteOrBackspace)
             {
-                textbox.Text = oldText;
+                textbox.Text = textbox.Text.Substring(0, oldText.Length);
                 if (oldText.Length >= 0)
                 {
                     textbox.SelectionStart = oldText.Length;
@@ -327,6 +327,7 @@ private static void Textbox_TextChanging(TextBox textbox, TextBoxTextChangingEve
                 if (string.IsNullOrEmpty(textbox.Text))
                 {
                     textbox.Text = displayText;
+                    return;
                 }
                 else
                 {