deps(api): upgrade pyfakefs to 6.x

`flagsmith-common[test-tools]` has required `pyfakefs>=6,<7` since
3.13.1, but the api capped it at `<6.0.0` and pinned `==5.7.4` via an
override. The override kept the api resolvable while leaving the
declared metadata contradictory, so consumers of flagsmith-api could not
resolve the two together.

Ref: #8368

Author: khvn26

api/pyproject.toml

@@ -115,7 +115,7 @@ dev = [
     "mypy-boto3-dynamodb>=1.33.0,<2.0.0",
     "mypy-boto3-s3>=1.36.0,<2.0.0",
     "pytest-structlog>=1.1,<2.0.0",
-    "pyfakefs>=5.7.4,<6.0.0",
+    "pyfakefs>=6,<7",
     "mypy>=1.15.0,<2.0.0",
     "djangorestframework-stubs>=3.15.2,<4.0.0",
     "boto3-stubs>=1.36.20,<2.0.0",
@@ -330,7 +330,6 @@ override-dependencies = [
     "pydantic==2.12.5",
     "pydantic-collections==0.6.0",
     "pydantic-core==2.41.5",
-    "pyfakefs==5.7.4",
     "pygithub==2.8.1",
     "pygments==2.19.2",
     "pylint==2.16.4",