minor #39823 [travis] use PHP 8.0 to patch return types and run deps=low (nicolas-grekas)

This PR was merged into the 4.4 branch.

Discussion
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 [travis] use PHP 8.0 to patch return types and run deps=low

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

(I need to verify that DebugClassLoader works as expected before merging)

Commits
-------

7f5ea78fb8 [travis] use PHP 8.0 to patch return types and run deps=low

Author: nicolas-grekas

Tests/Loader/GlobFileLoaderTest.php

@@ -40,5 +40,6 @@ class GlobFileLoaderWithoutImport extends GlobFileLoader
 {
     public function import($resource, $type = null, $ignoreErrors = false, $sourceResource = null, $exclude = null)
     {
+        return null;
     }
 }