Update "csdgm" schema_type value in HarvestSource model

[rshewitt]

User Story

In order to accurately describe the schema name, datagov wants to update the "csdgm" schema_type value in the HarvetSource model to "fgdc_1998"

Acceptance Criteria

[ACs should be clearly demoable/verifiable whenever possible. Try specifying them using BDD.]

• GIVEN the "csdgm" schema_type value in the HarvetSource model
  WHEN that value has changed to "fgdc_1998"
  THEN the model should accept "fgdc_1998" as a value for schema_type

Background

• investigation into this schema type resulted in #5087
• "csdgm" isn't descriptive enough and neither is "fgdc".

Security Considerations (required)

[Any security concerns that might be implicated in the change. "None" is OK, just be explicit here!]

Sketch

• update all references of "csdgm"
• update any tests/fixture/references of that value

[rshewitt]

this ticket is irrelevant if we don't support FGDC 1998

[btylerburton]

Related to #4899