Replaced ESI mentions with SSI when appropriate
minor #11531 [HttpKernel][FrameworkBundle] Replaced ESI mentions with SSI when appropriate (pgodel)

This PR was merged into the 2.6-dev branch.

Discussion
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[HttpKernel][FrameworkBundle] Replaced ESI mentions with SSI when appropriate

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

Commits
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7ee23c1 Replaced ESI mentions with SSI when appropriate