Improve Blazor project templates

[EdCharbeneau]

Improve Blazor project templates

Better a11y

• Overall better HTML semantics by changing the structure of the layout in markup. Visually the rendering is the same as .NET 7. A new Header.razor​​ component was added to accommodate the new layout.
• Collapsible navigation was rewritten to use a small bit of inline JavaScript instead of the improvised checkbox approach. The menu has proper aria-* attributes and sets focus to the first link when opened, per WCAG guidelines.
• Removed :focus​ override, this is bad practice for a11y.​

Bootstrap 5.3 / themes

• Updated to Bootstrap 5.3 and embraced Bootstrap throughout the sample layout. This eliminated ~100 lines of unnecessary CSS code and simplified the HTML too. Now MainLayout.razor.css only contains error ui​ CSS code.
• Enabled theming via Bootstrap 5.3's CSS variables (aka Custom Properties). This allows runtime theme changes and future proofs the code for Bootstrap vnext.
• Automatic light/dark mode detected and applied via user preferences. This is a no-code solution enabled by CSS variables. Developers can remove this feature by uncommenting a single line of code in App.razor
• Made use of inline svg​ icons. This enables light/dark​ themes to control icon color.
• Added --bl-*​ (opt-in) CSS variables to control the theme of the NavBar component. This allows devs to easily ditch the gradient if they want to customize the template.​

Code improvements

• Reduced complexity of CSS code throughout, app.css​ is much smaller and cleaner. Used updated semantics for mediaqueries. 
• Avoided CSS variables where code could be generated in  SampleContent == false​ templates.
• Add/Removed blazor-error-ui​ CSS code based on template interactivity. The markup was removed in the prior version, but not the CSS.

Fixes issue: #50927

[ghost]

Thanks for your PR, @EdCharbeneau. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.