RFR: build: update to JDK 15.0.1

[Erik Helin]

Hi all,

please review this patch that updates the JDK used for building Skara to 15.0.1.

Testing:
- [x] `make images`
- [x] `make test`

Thanks,
Erik

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Commit messages:
 - build: update to JDK 15.0.1

Changes: https://git.openjdk.java.net/skara/pull/969/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=969&range=00
  Stats: 15 lines in 4 files changed: 0 ins; 0 del; 15 mod
  Patch: https://git.openjdk.java.net/skara/pull/969.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/969/head:pull/969

PR: https://git.openjdk.java.net/skara/pull/969