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

• 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)
• 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

• 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.
• I have tested the feature as follows: attempted to reproduce again as described in KeY crashes after initial launch terminates unconventionally #3416 (comment), issue did not occur
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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