Add vector_math to package issue template (#187536)

Adds `vector_math` to the options for 1P packages in the issue template
now that it's been moved into flutter/core-packages

.github/ISSUE_TEMPLATE/08_first_party_packages.yml

diff --git a/.github/ISSUE_TEMPLATE/08_first_party_packages.yml b/.github/ISSUE_TEMPLATE/08_first_party_packages.yml
index 369d3bc..7f5d36e 100644
--- a/.github/ISSUE_TEMPLATE/08_first_party_packages.yml
+++ b/.github/ISSUE_TEMPLATE/08_first_party_packages.yml
@@ -65,6 +65,7 @@
         - two_dimensional_scrollables
         - url_launcher
         - vector_graphics
+        - vector_math
         - video_player
         - web_benchmarks
         - webview_flutter