Removing hardcoded values that limit widths and don't allow users to set custom styles that change size.

[MayaKirova]

Closes #15865

• Removing hardcoded defaultHeaderGroupMinWidth. Allow setting whatever width the user wants, without a limit, even if that width cannot be rendered correctly due to paddings or other styles.
• Removing related pipes.
• Moving styles to the igx-grid-header so that it matches perfectly with the cell.

Note: While you can set whatever width you want in case there is a mismatch between the user-set size and the actual size of the rendered cell i.e. you have set width to 20px (or % that resolves to 20px), but due to paddings the rendered cell is actually 48px (2x24px padding) then features like virtualization may not work as expected since they will not be aware of the actual size of the cell after styles are applied. It's up to the user to set valid size.

@dkamburov Applying the change for master only, since defaultHeaderGroupMinWidth is in the public API and removing it may be considered as breaking change.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them