minor symfony#40946 [DoctrineBridge] run tests against doctrine/persistence 2.1 (xabbuh)

This PR was merged into the 5.2 branch.

Discussion
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[DoctrineBridge] run tests against doctrine/persistence 2.1

| Q             | A
| ------------- | ---
| Branch?       | 5.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

Commits
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c8916f2 run tests against doctrine/persistence 2.1

Author: Nyholm

.travis.yml

@@ -281,6 +281,7 @@ install:
           elif [[ $deps = low ]]; then
               echo "$COMPONENTS" | parallel --gnu "tfold {} 'cd {} && $COMPOSER_UP --prefer-lowest --prefer-stable && $PHPUNIT_X'"
           else
+              composer require --no-update doctrine/persistence:2.1.*
               if [[ $PHP = 8.0* ]]; then
                   # add return types before running the test suite
                   sed -i 's/"\*\*\/Tests\/"//' composer.json