Use dynamic MSRV for oldstable CI

Instead of manually specifying the oldstable version in all our CI
scripts, it is now pulled from the `Cargo.toml` which simplifies the
update process.

The contributing guide has also been updated to not include the explicit
version and its wording has been loosened a bit to correctly represent
current maintenance practices.

.builds/freebsd.yml

@@ -24,8 +24,9 @@ tasks:
       cargo test
   - oldstable: |
       cd alacritty
-      rustup toolchain install --profile minimal 1.70.0
+      oldstable=$(cat alacritty/Cargo.toml | grep "rust-version" | sed 's/.*"\(.*\)".*/\1/')
-      rustup default 1.70.0
+      rustup toolchain install --profile minimal $oldstable
+      rustup default $oldstable
       cargo test
   - clippy: |
       cd alacritty

.builds/linux.yml

@@ -34,8 +34,9 @@ tasks:
       cargo test
   - oldstable: |
       cd alacritty
-      rustup toolchain install --profile minimal 1.70.0
+      oldstable=$(cat alacritty/Cargo.toml | grep "rust-version" | sed 's/.*"\(.*\)".*/\1/')
-      rustup default 1.70.0
+      rustup toolchain install --profile minimal $oldstable
+      rustup default $oldstable
       cargo test
   - clippy: |
       cd alacritty

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
         run: cargo test -p alacritty_terminal --no-default-features
       - name: Oldstable
         run: |
-          rustup default 1.70.0
+          rustup default $(cat alacritty/Cargo.toml | grep "rust-version" | sed 's/.*"\(.*\)".*/\1/')
           cargo test
       - name: Clippy
         run: |

CONTRIBUTING.md

@@ -42,8 +42,9 @@ and
 [easy](https://github.com/alacritty/alacritty/issues?q=is%3Aopen+is%3Aissue+label%3A%22D+-+easy%22)
 issues.
 
-Please note that the minimum supported version of Alacritty is Rust 1.70.0. All patches are expected
+You can find the minimum supported Rust version in Alacritty's manifest file
-to work with the minimum supported version.
+(`cat alacritty/Cargo.toml | grep "rust-version"`). Alacritty **must** always
+build with the MSRV and bumping it should be avoided if possible.
 
 Since `alacritty_terminal`'s version always tracks the next release, make sure that the version is
 bumped according to semver when necessary.