upgrade Twig to 2.15.3 or newer

[osma]

Reasons for creating this PR

Dependabot complains about a potential security issue in Twig <2.15.3. This PR upgrades Twig to ^2.15.3 (meaning: 2.15.3 or newer, but must still be 2.15)

AFAICT, the security issue doesn't really affect us, as we're not using templates where the name is derived from user input. But it's better to be sure and to get rid of warnings from vulnerability scanning tools.

Link to relevant issue(s), if any

• Closes #3

Description of the changes in this PR

• upgrade to Twig 2.15.3

Known problems or uncertainties in this PR

n/a

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Base: 71.17% // Head: 71.17% // No change to project coverage 👍

Coverage data is based on head (1c7d39b) compared to base (0bda0f5).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1370   +/-   ##
=========================================
  Coverage     71.17%   71.17%           
  Complexity     1650     1650           
=========================================
  Files            32       32           
  Lines          3788     3788           
=========================================
  Hits           2696     2696           
  Misses         1092     1092           

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