symfony

This PR was merged into the 2.7 branch.

Discussion
----------

[2.7][DX] Use constant message contextualisation for deprecations

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes

Since many projects are using this mechanism for deprecating feature, it is sometimes difficult to understand which vendor triggers a deprecation.
Sometimes we're using `since Symfony x.y` format, sometimes we are using `since x.y`. I propose to always use `since Symfony x.y` format.

Commits
-------

c2338cbd [2.7][DX] Use constant message contextualisation for deprecations