RFR: Avoid considering fixVersions that are not valid releases when calculating the hgupdate-sync label

[Robin Westberg]

There may be backport records that have fixVersions that should be ignored when calculating which of them should receive the hgupdate-sync label.

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Commit messages:
 - Avoid considering fixVersions that are not valid releases when calculating the hgupdate-sync label.

Changes: https://git.openjdk.java.net/skara/pull/1010/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1010&range=00
  Stats: 94 lines in 4 files changed: 31 ins; 9 del; 54 mod
  Patch: https://git.openjdk.java.net/skara/pull/1010.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1010/head:pull/1010

PR: https://git.openjdk.java.net/skara/pull/1010