Replace java.time APIs with kotlin(x).(date)time (#3001)

* Replace java.time APIs with kotlin(x).(date)time

kotlinx-datetime explicitly says in its README that it does not cover
i18n, so we have to stick with using their toJavaX converters wherever
we localise dates and times.

Yes, some of these replacements are... questionable. But I wanted to
try and maximise the replacement for now.

* Bump kotlinx-datetime to 0.8.0

* More kotlin.time/kotlinx-datetime replacements

* Remove redundant init block & make attributes val

* Replace new use of java.time.Instant

gradle/build-logic/src/main/kotlin/mihon/gradle/Commands.kt

@@ -1,10 +1,9 @@
 package mihon.gradle
 
 import org.gradle.api.Project
-import java.time.Instant
-import java.time.LocalDateTime
-import java.time.ZoneOffset
-import java.time.format.DateTimeFormatter
+import kotlin.time.Clock
+import kotlin.time.Duration.Companion.nanoseconds
+import kotlin.time.Instant
 
 // Git is needed in your system PATH for these commands to work.
 // If it's not installed, you can return a random value as a workaround
@@ -18,19 +17,18 @@ fun Project.getLatestCommitSha(): String {
     // return "1"
 }
 
-private val BUILD_TIME_FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss'Z'")
-
 /**
  * @param useLatestCommitTime If `true`, the build time is based on the timestamp of the last Git commit;
  *                          otherwise, the current time is used. Both are in UTC.
- * @return A formatted string representing the build time. The format used is defined by [BUILD_TIME_FORMATTER].
+ * @return An ISO 8601 formatted string representing the build time.
  */
 fun Project.getBuildTime(useLatestCommitTime: Boolean): String {
     return if (useLatestCommitTime) {
         val epoch = exec("git log -1 --format=%ct").toLong()
-        Instant.ofEpochSecond(epoch).atOffset(ZoneOffset.UTC).format(BUILD_TIME_FORMATTER)
+        Instant.fromEpochSeconds(epoch).toString()
     } else {
-        LocalDateTime.now(ZoneOffset.UTC).format(BUILD_TIME_FORMATTER)
+        val now = Clock.System.now()
+        (now - now.nanosecondsOfSecond.nanoseconds).toString()
     }
 }