Go/Java/C#: Rename to ActiveThreatModelSource

As part of adding support for threat-models to Python/JS (see
github#17203), we ran into some trouble
with name clashes.

Naming in existing languages supporting threat-models:
- `SourceNode` (for QL only modeling)
- `ThreatModelFlowSource` (for active sources from QL or data-extensions)

However, since we use `LocalSourceNode` in Python, and `SourceNode` in
JS (for local source nodes), it seems a bit confusing to follow the same
naming convention as other languages, and we had to come up with new names.

Initially I used `ThreatModelSource` for the "QL only modeling", but
that meant that we needed a new name to represent the active sources
coming from either QL or data-extensions... for this I came up with
`ActiveThreatModelSource`, and I really liked it. To me, it's much
clearer that this class only contains the currently active threat
model sources.

So to align languages, I got approval from @michaelnebel to rename the
existing classes.

csharp/ql/lib/semmle/code/csharp/security/dataflow/flowsources/FlowSources.qll

@@ -20,11 +20,18 @@ abstract class SourceNode extends DataFlow::Node {
 }
 
 /**
+ * DEPRECATED: Use `ActiveThreatModelSource` instead.
+ *
  * A class of data flow sources that respects the
  * current threat model configuration.
  */
-class ThreatModelFlowSource extends DataFlow::Node {
-  ThreatModelFlowSource() {
+deprecated class ThreatModelFlowSource = ActiveThreatModelSource;
+
+/**
+ * A data flow source that is enabled in the current threat model configuration.
+ */
+class ActiveThreatModelSource extends DataFlow::Node {
+  ActiveThreatModelSource() {
     exists(string kind |
       // Specific threat model.
       currentThreatModel(kind) and

go/ql/lib/semmle/go/security/FlowSources.qll

@@ -55,11 +55,18 @@ abstract class SourceNode extends DataFlow::Node {
 }
 
 /**
+ * DEPRECATED: Use `ActiveThreatModelSource` instead.
+ *
  * A class of data flow sources that respects the
  * current threat model configuration.
  */
-class ThreatModelFlowSource extends DataFlow::Node {
-  ThreatModelFlowSource() {
+deprecated class ThreatModelFlowSource = ActiveThreatModelSource;
+
+/**
+ * A data flow source that is enabled in the current threat model configuration.
+ */
+class ActiveThreatModelSource extends DataFlow::Node {
+  ActiveThreatModelSource() {
     exists(string kind |
       // Specific threat model.
       currentThreatModel(kind) and

java/ql/lib/semmle/code/java/dataflow/FlowSources.qll

@@ -42,11 +42,18 @@ abstract class SourceNode extends DataFlow::Node {
 }
 
 /**
+ * DEPRECATED: Use `ActiveThreatModelSource` instead.
+ *
  * A class of data flow sources that respects the
  * current threat model configuration.
  */
-class ThreatModelFlowSource extends DataFlow::Node {
-  ThreatModelFlowSource() {
+deprecated class ThreatModelFlowSource = ActiveThreatModelSource;
+
+/**
+ * A data flow source that is enabled in the current threat model configuration.
+ */
+class ActiveThreatModelSource extends DataFlow::Node {
+  ActiveThreatModelSource() {
     exists(string kind |
       // Specific threat model.
       currentThreatModel(kind) and