Normative: Use floor instead of truncate in epoch time getters

In Temporal.{Instant,ZonedDateTime}.epoch{S,Millis,Micros}econds, the
rounding should use floor instead of truncate semantics, in order to
always truncate towards the beginning of time, as .toString() and .round()
do.

Closes: #2423

spec/instant.html

@@ -162,7 +162,7 @@ <h1>get Temporal.Instant.prototype.epochSeconds</h1>
         1. Let _instant_ be the *this* value.
         1. Perform ? RequireInternalSlot(_instant_, [[InitializedTemporalInstant]]).
         1. Let _ns_ be _instant_.[[Nanoseconds]].
-        1. Let _s_ be truncate(ℝ(_ns_) / 10<sup>9</sup>).
+        1. Let _s_ be floor(ℝ(_ns_) / 10<sup>9</sup>).
         1. Return 𝔽(_s_).
       </emu-alg>
     </emu-clause>
@@ -177,7 +177,7 @@ <h1>get Temporal.Instant.prototype.epochMilliseconds</h1>
         1. Let _instant_ be the *this* value.
         1. Perform ? RequireInternalSlot(_instant_, [[InitializedTemporalInstant]]).
         1. Let _ns_ be _instant_.[[Nanoseconds]].
-        1. Let _ms_ be truncate(ℝ(_ns_) / 10<sup>6</sup>).
+        1. Let _ms_ be floor(ℝ(_ns_) / 10<sup>6</sup>).
         1. Return 𝔽(_ms_).
       </emu-alg>
     </emu-clause>
@@ -192,7 +192,7 @@ <h1>get Temporal.Instant.prototype.epochMicroseconds</h1>
         1. Let _instant_ be the *this* value.
         1. Perform ? RequireInternalSlot(_instant_, [[InitializedTemporalInstant]]).
         1. Let _ns_ be _instant_.[[Nanoseconds]].
-        1. Let _µs_ be truncate(ℝ(_ns_) / 10<sup>3</sup>).
+        1. Let _µs_ be floor(ℝ(_ns_) / 10<sup>3</sup>).
         1. Return ℤ(_µs_).
       </emu-alg>
     </emu-clause>

spec/zoneddatetime.html

@@ -313,7 +313,7 @@ <h1>get Temporal.ZonedDateTime.prototype.epochSeconds</h1>
         1. Let _zonedDateTime_ be the *this* value.
         1. Perform ? RequireInternalSlot(_zonedDateTime_, [[InitializedTemporalZonedDateTime]]).
         1. Let _ns_ be _zonedDateTime_.[[Nanoseconds]].
-        1. Let _s_ be truncate(ℝ(_ns_) / 10<sup>9</sup>).
+        1. Let _s_ be floor(ℝ(_ns_) / 10<sup>9</sup>).
         1. Return 𝔽(_s_).
       </emu-alg>
     </emu-clause>
@@ -328,7 +328,7 @@ <h1>get Temporal.ZonedDateTime.prototype.epochMilliseconds</h1>
         1. Let _zonedDateTime_ be the *this* value.
         1. Perform ? RequireInternalSlot(_zonedDateTime_, [[InitializedTemporalZonedDateTime]]).
         1. Let _ns_ be _zonedDateTime_.[[Nanoseconds]].
-        1. Let _ms_ be truncate(ℝ(_ns_) / 10<sup>6</sup>).
+        1. Let _ms_ be floor(ℝ(_ns_) / 10<sup>6</sup>).
         1. Return 𝔽(_ms_).
       </emu-alg>
     </emu-clause>
@@ -343,7 +343,7 @@ <h1>get Temporal.ZonedDateTime.prototype.epochMicroseconds</h1>
         1. Let _zonedDateTime_ be the *this* value.
         1. Perform ? RequireInternalSlot(_zonedDateTime_, [[InitializedTemporalZonedDateTime]]).
         1. Let _ns_ be _zonedDateTime_.[[Nanoseconds]].
-        1. Let _µs_ be truncate(ℝ(_ns_) / 10<sup>3</sup>).
+        1. Let _µs_ be floor(ℝ(_ns_) / 10<sup>3</sup>).
         1. Return ℤ(_µs_).
       </emu-alg>
     </emu-clause>