[jdk8u-dev] RFR: 8317374: Add Let's Encrypt ISRG Root X2

[Severin Gehwolf]

Backport so as to add the Let's Encrypt ISRG Root X2 CA. Applies clean after path shuffeling. Please review!

Testing:
- [ ] GHA (still running)
- [x] `jdk/test/security/infra/java/security/cert/CertPathValidator` and  `jdk/test/sun/security/lib/cacerts` jtreg tests.

-------------

Depends on: https://git.openjdk.org/jdk8u-dev/pull/397

Commit messages:
 - Backport e6f46a43268808d0cbbb3bb93c73aa8e4cbfad83

Changes: https://git.openjdk.org/jdk8u-dev/pull/398/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=398&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317374
  Stats: 39 lines in 3 files changed: 35 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/398.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/398/head:pull/398

PR: https://git.openjdk.org/jdk8u-dev/pull/398