Removed $min-contrast-ratio override to improve text contrast

[alexandrevryghem]

References

• Fixes Discussion: $min-contrast-ratio: 2 default in Bootstrap variables masks contrast failures #5606

Description

Removed the $min-contrast-ratio bootstrap override to ensure that all themes that use a light primary color still have a high enough text contrast.

Instructions for Reviewers

List of changes in this PR:

• Removed the $min-contrast-ratio override in _bootstrap_variables.scss & src/themes/dspace/styles/_theme_sass_variable_overrides.scss

Include guidance for how to test or review your PR.

• Override the primary color in your theme by setting this in src/themes/{theme}/styles/_theme_sass_variable_overrides.scss: $primary: #43515f
• Verify that without the fix the primary buttons have a failing contrast
  
• Run it again on this branch and verify that the primary button's text color is now dark enough to pass the WCAG AA threshold

Checklist

• My PR is created against the main branch of code (unless it is a backport or is fixing an issue specific to an older branch).
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