minor #12507 Updated the Welcome Page for Symfony 4.4 (javiereguiluz)

This PR was merged into the 4.4 branch.

Discussion
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Updated the Welcome Page for Symfony 4.4

The image is much bigger in size because of the background gradient. I already optimized as much as possible with image compression tools.

Commits
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32d1b54 Updated the Welcome Page for Symfony 4.4

Author: javiereguiluz