Cleaned up some issues in the maven code

[DavyLandman]

While reviewing the side-effect of case sensitive file systems I came across some issues that might be nice to fix.

The biggest one was the .properties file not getting closed as soon as possible.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 45%. Comparing base (22b7a02) to head (228f804).

Files with missing lines	Patch %	Lines
...c/org/rascalmpl/util/maven/MavenProxySelector.java	0%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              main   #2843   +/-   ##
=======================================
- Coverage       45%     45%   -1%     
+ Complexity    6718    6709    -9     
=======================================
  Files          841     841           
  Lines        67018   67020    +2     
  Branches     10012   10012           
=======================================
- Hits         30769   30768    -1     
- Misses       33863   33866    +3     
  Partials      2386    2386           

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