Bump django from 2.2.14 to 2.2.15 in /requirements (#15159)

Bumps [django](https://github.com/django/django) from 2.2.14 to 2.2.15.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/2.2.14...2.2.15)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/prod.txt

@@ -1,6 +1,6 @@
-Django==2.2.14 \
-    --hash=sha256:f2250bd35d0f6c23e930c544629934144e5dd39a4c06092e1050c731c1712ba8 \
-    --hash=sha256:edf0ecf6657713b0435b6757e6069466925cae70d634a3283c96b80c01e06191 # pyup: <3.0
+Django==2.2.15 \
+    --hash=sha256:91f540000227eace0504a24f508de26daa756353aa7376c6972d7920bc339a3a \
+    --hash=sha256:3e2f5d172215862abf2bac3138d8a04229d34dbd2d0dab42c6bf33876cc22323 # pyup: <3.0
 Babel==2.8.0 \
     --hash=sha256:1aac2ae2d0d8ea368fa90906567f5c08463d98ade155c0c4bfedd6a0f7160e38 \
     --hash=sha256:d670ea0b10f8b723672d3a6abeb87b565b244da220d76b4dba1b66269ec152d4