Fix startup crash if ~/.key/colors.json is not present

[FliegendeWurst]

Related Issue

This pull request fixes #3416.

Intended Change

This PR removes the fallback to the old .properties file in ColorSettings. Due to an issue in static initialization order, it is not possible to use the SettingsManager to (attempt to) read the old configuration file.

Type of pull request

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Refactoring (behaviour should not change or only minimally change)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] There are changes to the (Java) code
• [ ] There are changes to the taclet rule base
• [ ] There are changes to the deployment/CI infrastructure (gradle, github, ...)
• [ ] Other:

Ensuring quality

• [x] I made sure that introduced/changed code is well documented (javadoc and inline comments).
• [ ] I made sure that new/changed end-user features are well documented (https://github.com/KeYProject/key-docs).
• [ ] I added new test case(s) for new functionality.
• [x] I have tested the feature as follows: attempted to reproduce again as described in https://github.com/KeYProject/key/issues/3416#issuecomment-1985366875, issue did not occur
• [x] I have checked that runtime performance has not deteriorated.

Additional information and contact(s)

The contributions within this pull request are licensed under GPLv2 (only) for inclusion in KeY.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 37.85%. Comparing base (9ad6cc8) to head (4cf1755).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3439 +/- ## ========================================= Coverage 37.85% 37.85% Complexity 17042 17042 ========================================= Files 2082 2082 Lines 127290 127290 Branches 21441 21441 ========================================= Hits 48183 48183 Misses 73194 73194 Partials 5913 5913 ```

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