SEC-71727: fix bouncycastle version

[SerhiiKobezaCapgemini]

Summary

Upgrades org.bouncycastle:bcprov-jdk18on from 1.78.1 to 1.85 to fix 5 CVEs
reported by Snyk (SEC-71727, SEC-70987).

Changes

• app/build.gradle — switches from deprecated bcprov-jdk15to18:1.78 artifact
  to current bcprov-jdk18on:1.85
• guardian/build.gradle — adds resolutionStrategy.force to override the
  transitive path robolectric:4.14 → bcprov-jdk18on:1.78.1
• gradle.properties — adds bcprov-jdk18on to Jetifier ignorelist, since
  BouncyCastle 1.85 uses Java 25 class format which Jetifier cannot transform
  (no transformation is needed anyway — the library has no legacy support library references)

CVEs fixed

CVE	Severity	Type
CVE-2026-59650	High	Improper Input Validation
CVE-2026-8763	High	Improper Certificate Validation
CVE-2026-58062	High	Improper Certificate Validation
CVE-2026-59651	Medium	Inadequate Encryption Strength
CVE-2026-12185	Medium	Memory Allocation with Excessive Size Value

Closes SEC-71727, SEC-70987

Test plan

• ./gradlew :guardian:compileDebugJavaWithJavac passes
• ./gradlew :guardian:compileReleaseJavaWithJavac passes
• ./gradlew :guardian:test passes
• Snyk rescan shows no bcprov-jdk18on vulnerabilities