Bump flake8 to 7.1.1 and upgrade rustc and cargo versions

Lockfile v4 is being requested and version 1.78 of rustc
provides it.

Change-Id: I1da99d971a87772270d07a32cb806fb7769f727f

charmcraft.yaml

@@ -12,13 +12,13 @@ parts:
     - python3-dev
     - libffi-dev
     - libssl-dev
-    - rustc-1.76
-    - cargo-1.76
+    - rustc-1.78
+    - cargo-1.78
     - pkg-config
     override-build: |
-      # Note(mylesjp): Force build to use rustc-1.76
-      ln -s /usr/bin/rustc-1.76 /usr/bin/rustc
-      ln -s /usr/bin/cargo-1.76 /usr/bin/cargo
+      # Note(mylesjp): Force build to use rustc-1.78
+      ln -s /usr/bin/rustc-1.78 /usr/bin/rustc
+      ln -s /usr/bin/cargo-1.78 /usr/bin/cargo
       craftctl default
     build-snaps:
     - charm/latest/edge

tox.ini

@@ -64,7 +64,7 @@ commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
 basepython = python3
-deps = flake8==3.9.2
+deps = flake8==7.1.1
        git+https://github.com/juju/charm-tools.git
 commands = flake8 {posargs} src unit_tests \
     {toxinidir}/src/files/nagios/nrpe_check_ovn_certs.py \