Re-update SECURITY.md

[madolson]

We need to include our responsible disclosure agreement as well as our maintenance plan. I think this is something worth figuring out now. I would like to suggest we introduce a policy of maintaining all versions for at least 3 years. This will be a bit different from Redis, as we had some weird number of major/minor thing. The time will start when the release is first GA'd, so we will commit to maintaining our compatibility fork for 3 years.

[PenguRin]

That would be appealing for companies aswell and them potentially selecting and contributing primarily to this fork.

[Varpie]

After the name change to Valkey, I can see that the current SECURITY.md still uses the name PlaceholderKV.

[zuiderkwast]

Fixed?

[madolson]

I would like to suggest we introduce a policy of maintaining all versions for at least 3 years.

I was going to keep this open until this line was closed.