Deprecate advanced_search.

[rosiel]

GitHub Issue: Islandora/documentation#2001

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What does this Pull Request do?

Marks the Islandora submodule "Islandora Advanced Search" as deprecated, linking to the deprecation announcement.

How should this be tested?

Islandora Advanced Search will be marked "Deprecated" under the "Extend" tab.

Documentation Status

• Does this change existing behaviour that's currently documented? perhaps - another PR will follow.
• Does this change require new pages or sections of documentation? no
• Who does this need to be documented for? n/a
• Associated documentation pull request(s): ___ or documentation issue ___

Additional Notes:

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Interested parties

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