Add deprecation tags to customConfig entries in appD schema

[kriswest]

resolves #900

Sets the deprecated flag on customConfig elements in the appD spec:

prepares for #1006 expected post FDC3 2.1.

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[kriswest]

Un-assigning editor review and converting to a draft PR after meeting:

• Standard WG Meeting - May 25th, 2023 #1005

@lspiro-Tick42 @robmoffat and Alex@Citi all spoke up for a use-case for a vendor independent config that is readable by an app through the FDC3 API (the original intended purpose for this appD config field). There was a suggestion that it might be better understood if renamed to better reflect purpose and that a change to the FDC3 API should be proposed to make it accessible via standardized functions.

@kriswest suggested that this should be raised as a new issue and would be happy to collaborate on a proposal/PR. The topic should be addressed at a future FDC3 SWG meeting.

[kriswest]

Revived at #1013 (to be included in FDC3 2.1) as prep for:

• Custom application config, retrievable via a standardized function #1006
  which will use a different named element (likely to be applicationConfig) to better reflect its usage.

[kriswest]

@greyseer256 this deprecation is ready for your review now.