CA: Truncate notBefore and notAfter to second-level precision (#8319)

When generating the validity period of a to-be-issued certificate,
truncate the notBefore timestamp to second-level precision, trimming off
any nanoseconds which won't be represented in the final certificate. Do
the same for the notAfter, although this should be a no-op since only
whole numbers of seconds are used to compute it from the notBefore.

It's possible that this could cause some of the maxBackdate calculations
to fail, because truncation can cause the notBefore timestamp to move up
to (nearly) 1 second earlier. However, this only becomes a concern in
practice if maxBackdate is set to 10 seconds or less.

This results in cleaner logs, since Go only prints the fractional
seconds portion of a timestamp if it is non-zero:
https://go.dev/play/p/iAeSX3VMrJD

Fixes https://github.com/letsencrypt/boulder/issues/8318

issuance/cert.go

@@ -142,10 +142,10 @@ func (p *Profile) GenerateValidity(now time.Time) (time.Time, time.Time) {
 	// Don't use the full maxBackdate, to ensure that the actual backdate remains
 	// acceptable throughout the rest of the issuance process.
 	backdate := time.Duration(float64(p.maxBackdate.Nanoseconds()) * 0.9)
-	notBefore := now.Add(-1 * backdate)
+	notBefore := now.Add(-1 * backdate).Truncate(time.Second)
 	// Subtract one second, because certificate validity periods are *inclusive*
 	// of their final second (Baseline Requirements, Section 1.6.1).
-	notAfter := notBefore.Add(p.maxValidity).Add(-1 * time.Second)
+	notAfter := notBefore.Add(p.maxValidity).Add(-1 * time.Second).Truncate(time.Second)
 	return notBefore, notAfter
 }