[FrameworkBundle] Redirect server output to /dev/null in case no verbosity is needed
minor #11122 [2.5][FrameworkBundle] Redirect server output to /dev/null in case no verbosity is needed (romainneutron)

This PR was merged into the 2.5 branch.

Discussion
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[2.5][FrameworkBundle] Redirect server output to /dev/null in case no verbosity is needed

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

This should reduce I/Os

Commits
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6d9fd62 [FrameworkBundle] Redirect server output to /dev/null in case no verbosity is needed