DRYD-1978: Increase contrast for disabled mini buttons

[mikejritter]

What does this do?
Changes the contrast for disabled MiniButton elements

Why are we doing this? (with JIRA link)
Jira: https://collectionspace.atlassian.net/browse/DRYD-1978

This fixes an accessibility violation in the pager and other components of cspace-ui. By using the existing textDark, we should see a contrast ratio of ~7.

How should this be tested? Do these changes have associated tests?

• Rebuild cspace-input
• Deploy the updated cspace-input in your cspace-ui (npm link worked for me; I've had trouble with hooks breaking before though).
• Using the advanced search, search on any record type
• See that the pager component should have the proper contrast ratio

Dependencies for merging? Releasing to production?
None.

Has the application documentation been updated for these changes?
n/a

Did someone actually run this code to verify it works?
@mikejritter tested against a local CollectionSpace