Fix for the broken "earliest date" (#9309)

[landreev]

What this PR does / why we need it:

A trivial fix for the embarrassing OAI server bug in 12.1

Which issue(s) this PR closes:

Closes #9309

Special notes for your reviewer:

Suggestions on how to test this:

(I have synced with develop and resolved the merge conflict; it is indeed ready for QA - but please see the note below)

One important note: the PR #9310, that has already been merged, provided a parallel fix for the worst manifestation of this bug, the part where incremental harvest requests were being rejected as described in the issue. This means you can no longer test that symptoms on develop branch in its current form (although it's still present in prod. and demo). (to clarify: it "provided a parallel fix" because the problem had also been fixed in the XOAI library, and in that PR we switched to the latest version of it).

We still need this PR, because it fixes the remaining part of the bug. Namely, the "earliest record in the repository" time stamp is still advertised incorrectly, and this can still be observed in develop. For example, looking at https://dataverse-internal.iq.harvard.edu/oai?verb=Identify, the earliest record's date is shown to be the "epoch", i.e. 1970-01-01:

<OAI-PMH xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/ http://www.openarchives.org/OAI/2.0/OAI-PMH.xsd">
<responseDate>2023-01-30T17:15:05Z</responseDate>
<request verb="Identify">https://dataverse-internal.iq.harvard.edu/oai</request>
<Identify>
<repositoryName>Dataverse Internal Dataverse OAI Archive</repositoryName>
<baseURL>https://dataverse-internal.iq.harvard.edu/oai</baseURL>
<protocolVersion>2.0</protocolVersion>
<adminEmail>root@dataverse-internal.iq.harvard.edu</adminEmail>
<earliestDatestamp>1970-01-01T00:00:00Z</earliestDatestamp>
<deletedRecord>transient</deletedRecord>
<granularity>YYYY-MM-DDThh:mm:ssZ</granularity>
<compression>gzip</compression>
<compression>deflate</compression>
</Identify>
</OAI-PMH>

(and in prod. and demo, it is still being reported as the time of the last Payara restart ☹️ )
WIth the fix in this branch, it will change to the actual timestamp on the earliest record in the OAI sets on the server.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Is there a release notes update needed for this change?:

Additional documentation:

[mreekie]

Daily:

• This has been raised by @landreev and has been ok'd for the next sprint.
• @landreev will size this and Stefano has Ok'd this to put at the top of our thesprint we start tomorrow.

[landreev]

@mreekie

* @landreev will size this and Stefano has Ok'd this to put at the top of our thesprint we start tomorrow.

I believe this is an honest 10 at most, review and QA combined. It's straightforward to reproduce, just as straightforward to verify that the fix does indeed fix it.
I would size it at even lower, but I'm calling it a 10 because there's going to be more than 1 person involved (somebody will have to review it and Kevin will have to test it).

[coveralls]

Coverage: 19.936% (-0.003%) from 19.938% when pulling a7a2985 on 9309-oai-earliest-date into dea8eb5 on develop.