[Dlang-internal] Making DIP25 the default

[Dicebot via Dlang-internal]

It has been a very long time since -dip25 CLI switch was introduced.
More importantly, it is a requirement for coming DIP1000, which means
that turning it into normal behavior must be prioritized.

I have quickly checked current DMD sources and this seems to be the only
place affected by the switch:

```
  // escape.d
  /* Check for returning a ref variable by 'ref', but should be 'return ref'
   * Infer the addition of 'return', or set result to be the offending
expression.
   */
  if (global.params.useDIP25 &&
     (v.storage_class & (STCref | STCout)) &&
    !(v.storage_class & (STCreturn | STCforeach)))
  {
```

Looks like it is relatively simple to enable that block unconditionally
and turn errors inside it into deprecations.

@Martin would be OK with adding this change to coming release? That way
we will have more time to fix any issues before DIP1000 comes into play.

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