Theming: Make all Palette properties virtual

[TDroogers]

Description

Followup for #5676
Mark all Pelette variables virtual so you can inherent Palette in your own object like I did for PaletteDark.
Also marked Pelette Obsolete, mostly to inform people about the existence of PaletteDark

How Has This Been Tested?

Visual, don't know how this could be tested with unit tests.
Tested if Pelette can be marked abstract for the doc project and changed doc to make sure it works.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (fe5273d) 91.17% compared to head (a0e94f0) 91.17%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #6519   +/-   ##
=======================================
  Coverage   91.17%   91.17%           
=======================================
  Files         392      392           
  Lines       14836    14836           
=======================================
  Hits        13527    13527           
  Misses       1309     1309           

Impacted Files	Coverage Δ
src/MudBlazor/Themes/Models/Palette.cs	78.08% <100.00%> (ø)
src/MudBlazor/Themes/MudTheme.cs	100.00% <100.00%> (ø)

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[TDroogers]

@henon A small improvement on my old PR #5676, decided to LightPallete like it was discussed in that PR.

[henon]

I totally agree. We'll make it abstract with v7