use `dist/release` KEYS

[kevinjqliu]

Deprecate the use of

• https://dist.apache.org/repos/dist/dev/iceberg/KEYS in favor of
• https://downloads.apache.org/iceberg/KEYS which is an alias for https://dist.apache.org/repos/dist/release/iceberg/KEYS

devlist discussion: https://lists.apache.org/thread/8j41w4y2jx6r3ybj0o82bfyn0npmhgx2

[kevinjqliu]

Updated to use downloads whenever possible, except when uploading new keys

[RussellSpitzer]

Thanks @kevinjqliu for the PR and @zeroshade, @Fokko for review!