refactor(core): Replace vendored ISO8601Utils with java.time

[runningcode]

Summary

• Remove the vendored ISO8601Utils class (adapted from FasterXML Jackson Databind) and replace with java.time APIs in DateUtils
• Keep a compact legacy fallback using Calendar for Android API 21-25 where java.time is unavailable without core library desugaring
• Runtime detection via Class.forName("java.time.Instant") selects the implementation, following the existing SentryInstantDate/SentryAutoDateProvider pattern

Notes on Android compatibility

java.time is only available on Android API 26+. Customers on API 21-25 without core library desugaring enabled would crash if we used java.time unconditionally. Since desugaring is opt-in (requires isCoreLibraryDesugaringEnabled = true + adding the desugar_jdk_libs dependency), and the SDK does not require it, we use runtime detection to fall back to a manual parser/formatter on older devices.

Breaking change

ISO8601Utils was in the .api surface (though marked @ApiStatus.Internal). Removing it is a binary-incompatible change for anyone who happened to depend on it directly.

🤖 Generated with Claude Code

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.40.0 (1)	release

⚙️ sentry-android Build Distribution Settings

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Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	305.96 ms	367.40 ms	61.44 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
2195398	319.02 ms	342.38 ms	23.36 ms
d364ace	411.72 ms	430.81 ms	19.10 ms
d15471f	286.65 ms	314.68 ms	28.03 ms
ff8eea4	313.42 ms	337.08 ms	23.66 ms
17a0955	372.53 ms	446.70 ms	74.17 ms
23d6b12	354.10 ms	408.38 ms	54.28 ms
cf708bd	408.35 ms	458.98 ms	50.63 ms
6edfca2	305.52 ms	432.78 ms	127.26 ms
72020f8	312.32 ms	370.94 ms	58.62 ms
6b019b7	343.31 ms	417.23 ms	73.91 ms

App size

Revision	Plain	With Sentry	Diff
2195398	0 B	0 B	0 B
d364ace	1.58 MiB	2.11 MiB	539.75 KiB
d15471f	1.58 MiB	2.13 MiB	559.54 KiB
ff8eea4	1.58 MiB	2.28 MiB	718.64 KiB
17a0955	1.58 MiB	2.10 MiB	533.20 KiB
23d6b12	1.58 MiB	2.10 MiB	532.31 KiB
cf708bd	1.58 MiB	2.11 MiB	539.71 KiB
6edfca2	1.58 MiB	2.13 MiB	559.07 KiB
72020f8	1.58 MiB	2.19 MiB	620.21 KiB
6b019b7	0 B	0 B	0 B