Don't use the deprecated spaceless filter

[javiereguiluz]

This fixes the following deprecations that are making CI to fail:

Remaining indirect deprecation notices (3)

  1x: Since twig/twig 3.12: Twig Filter "spaceless" is deprecated in /home/runner/work/docs-builder/docs-builder/src/Templates/default/html/toc.html.twig at line 1.
    1x in BuildDocsCommandTest::testBuildDocsDefault from App\Tests\Command

  1x: Since twig/twig 3.12: Twig Filter "spaceless" is deprecated in /home/runner/work/docs-builder/docs-builder/src/Templates/default/html/link.html.twig at line 1.
    1x in BuildDocsCommandTest::testBuildDocsDefault from App\Tests\Command

  1x: Since twig/twig 3.12: Twig Filter "spaceless" is deprecated in /home/runner/work/docs-builder/docs-builder/src/Templates/default/html/figure.html.twig at line 4.
    1x in IntegrationTest::testParseUnitBlock from SymfonyDocsBuilder\Tests