Bump min Bazel version to 6.3 (#2150)

A future Bazel version will contain a breaking API change.
#2072 is preparing the
`rules_rust` codebase for it, however it relies on Bazel API changes
present in `6.3.0` and not `6.0.0`. As per discussion there
#2072 (comment),
looks like maintainers are fine with bumping the minimum Bazel version
to `6.3.0`.

.bazelci/presubmit.yml

@@ -221,7 +221,7 @@ tasks:
   ubuntu1804:
     name: "Min Bazel Version"
     # If updating the minimum bazel version, please also update /docs/index.md
-    bazel: "6.0.0"
+    bazel: "6.3.0"
     platform: ubuntu1804
     # Disable use of `instrumented_files_info.metadata_files` until Bazel 6.3 is the min version
     shell_commands:
@@ -232,7 +232,7 @@ tasks:
     post_shell_commands: *coverage_validation_post_shell_commands
   ubuntu1804_with_aspects:
     name: "Min Bazel Version With Aspects"
-    bazel: "6.0.0"
+    bazel: "6.3.0"
     platform: ubuntu1804
     # Disable use of `instrumented_files_info.metadata_files` until Bazel 6.3 is the min version
     shell_commands:

docs/index.md

@@ -93,7 +93,7 @@ Failure to do so will result in rules attempting to match a `stable` toolchain w
 
 ## Supported bazel versions
 
-The oldest version of Bazel the `main` branch is tested against is `6.0.0`. Previous versions may still be functional in certain environments, but this is the minimum version we strive to fully support.
+The oldest version of Bazel the `main` branch is tested against is `6.3.0`. Previous versions may still be functional in certain environments, but this is the minimum version we strive to fully support.
 
 We test these rules against the latest rolling releases of Bazel, and aim for compatibility with them, but prioritise stable releases over rolling releases where necessary.