docs: Mention that most linux infra is on jammy now

Currently, the vast majority of chromium builds run on jammy. And
after https://crrev.com/c/4652993, any new builder will automatically
be running on jammy.

So jammy is now the official default for infra jobs. This updates
the docs to mention that, and clarifies them a little since the
statement about "most development" might apply to more than just CI
builds, and we can't be as confident about the OS breakdown outside
of our infra.

Bug: 1412588
Change-Id: I751abf251db9758c0b6c12733a2e06f7a5e3e5c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4654783
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1163837}

docs/linux/build_instructions.md

@@ -18,8 +18,10 @@ Are you a Google employee? See
 *   You must have Git and Python v3.6+ installed already (and `python3` must point
     to a Python v3.6+ binary).
 
-Most development is done on Ubuntu (currently 18.04, Bionic Beaver). There are
-some instructions for other distros below, but they are mostly unsupported.
+
+Most development is done on Ubuntu (Chromium's build infrastructure currently
+runs 22.04, Jammy Jellyfish). There are some instructions for other distros
+below, but they are mostly unsupported.
 
 ### Docker requirements