Switch from bazeliskrc to bazelversion (#4612)

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.

.bazelversion

@@ -0,0 +1 @@
+7.3.0

MODULE.bazel

@@ -7,24 +7,12 @@
 `MODULE.bazel.lock` may change locally when `bazel` is executed. This means one
 of:
 
-1.  `MODULE.bazel` is changing.
+1.  An input is changing, typically `MODULE.bazel` or `.bazelversion`.
     -   Running `bazel mod deps` provides a canonical update to
         `MODULE.bazel.lock`; include the changes.
     -   GitHub test actions may also identify platform-specific lockfile
         updates.
-2.  The `bazel` version doesn't match the version used to generate the
-    `MODULE.bazel.lock`.
-    -   Bazel releases can change the lock file contents.
-    -   We use Bazelisk in order to keep everyone using the same version:
-        https://bazel.build/install/bazelisk
-        -   Developers are encouraged to alias `bazel` to `bazelisk` for
-            ease-of-use; some setups, like Homebrew, will include that with a
-            `bazelisk` install.
-        -   Contribution tools documentation includes `bazelisk` setup:
-            https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/contribution_tools.md
-    -   Make sure `bazelisk --version` (or `bazel --version`) matches the
-        version in `.bazeliskrc`.
-3.  The host platform hasn't yet been added to the lock file.
+2.  The host platform hasn't yet been added to the lock file.
     -   Platforms tested with GitHub actions are kept up-to-date. Other
         platforms may fall out of sync due to `bazel` or dependency changes,
         and should be updated with a PR the same way a platform is added.

docs/project/contribution_tools.md

@@ -169,7 +169,7 @@ These tools are essential for work on Carbon.
 -   Main tools
     -   [Bazel](https://www.bazel.build/)
         -   [Bazelisk](https://docs.bazel.build/versions/master/install-bazelisk.html):
-            Downloads and runs the [configured Bazel version](/.bazeliskrc).
+            Downloads and runs the [configured Bazel version](/.bazelversion).
     -   [Clang](https://clang.llvm.org/) and [LLVM](https://llvm.org/)
         -   NOTE: Most LLVM 14+ installs should build Carbon. If you're having
             issues, see