[FrameworkBundle] set a default value for gc_probability
minor #10366 [FrameworkBundle] set a default value for gc_probability (fabpot)

This PR was merged into the 2.5-dev branch.

Discussion
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[FrameworkBundle] set a default value for gc_probability

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | refs #10349
| License       | MIT
| Doc PR        | n/a

This sets the `gc_probability` to 1 by default (which is the default value in PHP). That should help mitigate #10349.

Commits
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7e1bdd7 [FrameworkBundle] set a default value for gc_probability