RFR: forge: use /app/installations/ endpoint for GitHub access tokens

[Erik Helin]

Hi all,

please review this small patch that changes the REST endpoint for creating GitHub access tokens from
`/installations/:installation_id/access_tokens` to `/app/installations/:installation_id/access_tokens`. GitHub
[announced in
April](https://developer.github.com/changes/2020-04-15-replacing-create-installation-access-token-endpoint/) that they
are deprecating the `/installations/:installation_id/access_tokens` endpoint and that applications should migrate to
the `/app/installations/:installation_id/access_tokens` endpoint which features identical semantics.

Thanks,
Erik

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Commit messages:
 - forge: use /app/installations/ endpoint for GitHub access tokens

Changes: https://git.openjdk.java.net/skara/pull/703/files
 Webrev: https://webrevs.openjdk.java.net/skara/703/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/703.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/703/head:pull/703

PR: https://git.openjdk.java.net/skara/pull/703