Fix isIndexable() DateTimeImmutable bug

[maltehuebner]

Summary

• Fix bug where DateTimeImmutable::sub() return value was discarded
• Method was always comparing against "now" instead of "one week ago"

Problem

// Before: sub() return value lost
$dateTime = new \DateTimeImmutable();
$dateTime->sub(new \DateInterval('P1W')); // returns new object, ignored
return $dateTime >= $this->dateTime;      // always compares with "now"

Test plan

• Verify that isIndexable() correctly returns true only for data older than 1 week

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