Adding note about the PdoSessionHandler BC break
Adding note about known BC issues
minor #12890 Adding note about the PdoSessionHandler BC break (weaverryan)

This PR was merged into the 2.6 branch.

Discussion
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Adding note about the PdoSessionHandler BC break

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12833
| License       | MIT
| Doc PR        | symfony/symfony-docs#4609

Hi guys!

This adds an UPGRADE note about the PdoSessionHandler BC break in 2.6. I also added a section on top, which might be more controversial that highlights the BC breaks. That's because this document is long, and much of it is telling you about deprecated features. But what I first want to know is - what will break?

I'm also still interested in maybe adding the old PdoSessionHandler to 2.6 and allowing you to use that one to keep BC (still a BC break, but you could choose to use the old one).

Thanks!

Commits
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adfdad9 Adding note about known BC issues
42950e9 Adding note about the PdoSessionHandler BC break