[Test] Changed the script to use gcc8 (#34467)

Changed missing piece of https://github.com/grpc/grpc/pull/34444

tools/run_tests/run_tests.py

@@ -560,8 +560,8 @@ class CLanguage(object):
 
         if compiler == "default" or compiler == "cmake":
             return ("debian11", [])
-        elif compiler == "gcc7":
-            return ("gcc_7", [])
+        elif compiler == "gcc8":
+            return ("gcc_8", [])
         elif compiler == "gcc10.2":
             return ("debian11", [])
         elif compiler == "gcc10.2_openssl102":
@@ -1714,7 +1714,7 @@ argp.add_argument(
     "--compiler",
     choices=[
         "default",
-        "gcc7",
+        "gcc8",
         "gcc10.2",
         "gcc10.2_openssl102",
         "gcc12",

tools/run_tests/run_tests_matrix.py

@@ -354,7 +354,7 @@ def _create_portability_test_jobs(
 
     # portability C and C++ on x64
     for compiler in [
-        "gcc7",
+        "gcc8",
         # 'gcc10.2_openssl102', // TODO(b/283304471): Enable this later
         "gcc12",
         "gcc12_openssl309",