[FS-509] 28-day Proposal Hold Time

[mdimjasevic]

Rohan asked to keep proposal hold time consistent with message hold time, which is 28 days by default. The PR updates the value accordingly.

Tracked by https://wearezeta.atlassian.net/browse/FS-509.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information (details):
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