resource+tfsdk: Remove tfsdk package schema types and functions

Reference: #132
Reference: #326
Reference: #491

Refer to the following pull request descriptions for migration information about migrating to the split schema packages:

- `datasource/schema`: #546
- `provider/schema`: #553
- `resource/schema`: #558
- `provider/metaschema`: #562

Author: bflad

.changelog/pending.txt

@@ -0,0 +1,31 @@
+```release-note:breaking-change
+resource: The `RequiresReplace()` plan modifier has been removed. Use a type-specific plan modifier instead, such as `resource/schema/stringplanmodifier.RequiresReplace()` or `resource/schema/stringplanmodifier.RequiresReplaceIfConfigured()`
+```
+
+```release-note:breaking-change
+resource: The `RequiresReplaceIf()` plan modifier has been removed. Use a type-specific plan modifier instead, such as `resource/schema/stringplanmodifier.RequiresReplaceIf()`
+```
+
+```release-note:breaking-change
+resource: The `Resource` type `GetSchema` method has been removed. Use the `Schema` method instead.
+```
+
+```release-note:breaking-change
+resource: The `UseStateForUnknown()` plan modifier has been removed. Use a type-specific plan modifier instead, such as `resource/schema/stringplanmodifier.UseStateForUnknown()`
+```
+
+```release-note:breaking-change
+tfsdk: The `Attribute`, `Block`, and `Schema` types have been removed. Use the similarly named types in the `datasource/schema`, `provider/schema`, and `resource/schema` packages instead.
+```
+
+```release-note:breaking-change
+tfsdk: The `AttributePlanModifier` interface has been removed. Use the type-specific plan modifier interfaces in the `resource/schema/planmodifier` package instead.
+```
+
+```release-note:breaking-change
+tfsdk: The `AttributeValidator` interface has been removed. Use the type-specific validator interfaces in the `schema/validator` package instead.
+```
+
+```release-note:breaking-change
+tfsdk: The `ListNestedAttributes`, `MapNestedAttributes`, `SetNestedAttributes`, and `SingleNestedAttributes` functions have been removed. Use the similarly named types in the `datasource/schema`, `provider/schema`, and `resource/schema` packages instead.
+```