MudTable: Fix inconsistent row border thickness

[dennml]

Description

Change MudTable _table.scss to have border-collapse: separate; instead of border-collapse: collapse;
This results in consistent display of borders.

border-collapse: separate; is already used if the table is set to fixed header FixedHeader="true" and those tables do not have the issue with inconsistent border thickness.

Update 1: removed all instances of have border-collapse: separate; which is redundant as that is the default value

Fixes #5822

How Has This Been Tested?

Visual tests with Docs and API documentation that also use MudTable as per following before and after screenshots.
In Docs, toggled all options on and off and observed no issues.

Before 1:

After 1:

Before 2:

After 2:

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

Base: 91.50% // Head: 91.49% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (65b0203) compared to base (2f65df6).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #5823      +/-   ##
==========================================
- Coverage   91.50%   91.49%   -0.01%     
==========================================
  Files         381      381              
  Lines       14943    14943              
==========================================
- Hits        13673    13672       -1     
- Misses       1270     1271       +1     

Impacted Files	Coverage Δ
...r/Components/Autocomplete/MudAutocomplete.razor.cs	92.43% <0.00%> (-0.43%)	⬇️

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[mikes-gh]

@JonBunator Removing defaults seems like a good move (less code). Are you happy with the new changes