docs: mention that that Rust 1.76 is required to install from source

We have our MSRV recorded in `Cargo.toml`, but at least with Rust
versions < 1.71, the build fails before the MSRV is checked (IIUC),
and you get a cryptic message like this:

```
error: Package jj-lib v0.22.0 (<some path>) does not have feature git2.
```

The message is clearer from 1.71, but that doesn't help users who have
an older version. Let's mention the MSRV everywhere we tell the user
to run `cargo` in the installation instructions.

Cargo.toml

@@ -7,7 +7,7 @@ members = ["cli", "lib", "lib/gen-protos", "lib/proc-macros", "lib/testutils"]
 [workspace.package]
 version = "0.22.0"
 license = "Apache-2.0"
-rust-version = "1.76"  # NOTE: remember to update CI, contributing.md, changelog.md, and flake.nix
+rust-version = "1.76" # NOTE: remember to update CI, contributing.md, changelog.md, install-and-setup.md, and flake.nix
 edition = "2021"
 readme = "README.md"
 homepage = "https://github.com/martinvonz/jj"

docs/install-and-setup.md

@@ -30,8 +30,9 @@ be compiled from the same source code.
 
 #### From Source
 
-First make sure that you have the `libssl-dev`, `openssl`, `pkg-config`, and
-`build-essential` packages installed by running something like this:
+First make sure that you have a Rust version >= 1.76 and that the `libssl-dev`,
+`openssl`, `pkg-config`, and `build-essential` packages installed by running
+something like this:
 
 ```shell
 sudo apt-get install libssl-dev openssl pkg-config build-essential
@@ -109,7 +110,7 @@ emerge -av dev-vcs/jj
 
 #### From Source, Vendored OpenSSL
 
-You may need to run:
+First make sure that you have a Rust version >= 1.76. You may also need to run:
 
 ```shell
 xcode-select --install
@@ -132,8 +133,9 @@ cargo install --features vendored-openssl -locked --bin jj jj-cli
 
 #### From Source, Homebrew OpenSSL
 
-You will need [Homebrew](https://brew.sh/) installed. You may then need to run
-some or all of these:
+First make sure that you have a Rust version >= 1.76. You will also need
+[Homebrew](https://brew.sh/) installed. You may then need to run some or all of
+these:
 
 ```shell
 xcode-select --install
@@ -178,7 +180,7 @@ sudo port install jujutsu
 
 ### Windows
 
-Run either:
+First make sure that you have a Rust version >= 1.76. Now run either:
 
 ```shell
 # To install the *prerelease* version from the main branch