Bump pip from 25.3 to 26.0 in /kokoro/gcp_ubuntu

Bumps [pip](https://github.com/pypa/pip) from 25.3 to 26.0.
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@25.3...26.0)

---
updated-dependencies:
- dependency-name: pip
  dependency-version: '26.0'
  dependency-type: direct:production
...

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

Author: dependabot[bot]

kokoro/gcp_ubuntu/requirements.txt

@@ -1,5 +1,5 @@
 # pip requirements generated from pypi
-pip==25.3 --hash=sha256:8d0538dbbd7babbd207f261ed969c65de439f6bc9e5dbd3b3b9a77f25d95f343 --hash=sha256:9655943313a94722b7774661c21049070f6bbb0a1516bf02f7c8d5d9201514cd
+pip==26.0 --hash=sha256:3ce220a0a17915972fbf1ab451baae1521c4539e778b28127efa79b974aff0fa --hash=sha256:98436feffb9e31bc9339cf369fd55d3331b1580b6a6f1173bacacddcf9c34754
 virtualenv==20.36.1 --hash=sha256:575a8d6b124ef88f6f51d56d656132389f961062a9177016a50e4f507bbcc19f --hash=sha256:8befb5c81842c641f8ee658481e42641c68b5eab3521d8e092d18320902466ba
 distlib==0.3.8 --hash=sha256:034db59a0b96f8ca18035f36290806a9a6e6bd9d1ff91e45a7f172eb17e51784
 filelock==3.20.3 --hash=sha256:18c57ee915c7ec61cff0ecf7f0f869936c7c30191bb0cf406f1341778d0834e1 --hash=sha256:4b0dda527ee31078689fc205ec4f1c1bf7d56cf88b6dc9426c4f230e46c2dce1