remap-lang: serialize timestamp value in RFC3339

[fanatid]

Initial discussion: #5653 (comment)

[binarylogic]

cc @JeanMertz and @FungusHumungus for input/approval.

[JeanMertz]

I agree. This was an oversight on my part, as I expected DateTime<UTC>'s Display impl to actually result in an RFC3339 formatted string.