Switch from bazeliskrc to bazelversion

[jonmeow]

The only documentation I can find for .bazelversion is in https://github.com/bazelbuild/bazelisk/blob/master/README.md. However, bazel will error out if it doesn't match the .bazelversion. For example:

╚╡/usr/bin/bazel build :all
ERROR: The project you're trying to build requires Bazel 7.3.0 (specified in [elided]/carbon-lang/.bazelversion), but it wasn't found in /usr/bin.

Switching to this because the MODULE.bazel requires tighter version pinning in order to avoid churn, and this should help catch mistakes early.

In MODULE.bazel, demote mention of the version issue because it should only really occur now when the version is being deliberately changed, so the connection should be more apparent.

[chandlerc]

Nice, does seem to be a much nicer flow here. And as you say, with module files, it's an important constraint.

I'm nervous about the complete lack of docs, but it seems to be intended to be supported and there is a 4 year old bug to document it: bazelbuild/bazel#11264

Not sure if you want to link that anywhere, but generally LG.

[jonmeow]

I can't put any comments in the file or it doesn't work. Other than that, I couldn't think of a really good spot to put documents, and it hadn't seemed hard enough to figure out (put differently, I think the current documentation is pretty similar to the .bazeliskrc, I was mainly commenting for the PR).

[jonmeow]

Note, part of my thought is that bazelversion nuance feels a little more detailed "how the infrastructure works" than what's currently in contribution tools, which is more "here's how to get set up".