docs: update graph per feedback

Add link to image (test) and update image and raw mermaid file.

docs/core-tasks/upgrading-an-application.mdx

@@ -68,9 +68,9 @@ For a working example, see the following pull requests from the 21-Points Health
 
 Here is how the upgrade process works graphically (read the sections below to have a textual explanation):
 
-![GitGraph](/upgrading-an-application/images/upgrade_gitgraph.png)
+![Git graph showing each invocation of 'npx jhipster upgrade' and the commits on jhipster_upgrade branch](/upgrading-an-application/images/upgrade_gitgraph.png)
 
-(Mermaid graph [raw mermaid file](/upgrading-an-application/jhipster_upgrade.mermaid))
+[Raw .mermaid file](/upgrading-an-application/jhipster_upgrade.mermaid)
 
 Please note that the `jhipster_upgrade` branch will be created orphan on your project, although it doesn't display correctly on the above graph.
 

static/upgrading-an-application/jhipster_upgrade.mermaid

@@ -1,22 +1,42 @@
-%%{init: { 'logLevel': 'debug', 'theme': 'neutral' } }%%
+%%{init: { 'logLevel': 'debug', 'theme': 'default', 'themeVariables': {
+    'git0': '#004677',
+    'git1': '#236fa4',
+    'git2': '#236fa4',
+    'gitBranchLabel1': '#ffffff',
+    'gitBranchLabel2': '#ffffff'
+} } }%%
 gitGraph LR:
+    branch jhipster_upgrade
+    branch "each 'npx jhipster upgrade'"
+    checkout main
     commit id: "chore: initial commit" tag: "1.0.0"
     commit id: "feat: controller" tag: "1.1.0"
-    branch jhipster_upgrade
     checkout jhipster_upgrade
-    commit id: "feat(8.0.0): npx jhipster upgrade"
+    commit id: "7.9.4"
+    checkout main
+    merge jhipster_upgrade type: HIGHLIGHT
+    checkout jhipster_upgrade
+    commit id: "8.0.0"
     checkout main
     merge jhipster_upgrade tag: "1.2.0"
     commit id: "fix: cool patch" tag: "1.2.1"
     commit id: "docs: update variable"
     checkout jhipster_upgrade
-    commit id: "feat(8.1.0): npx jhipster upgrade"
+    commit id: "8.0.0'"
+    checkout main
+    merge jhipster_upgrade type: HIGHLIGHT
+    checkout jhipster_upgrade
+    commit id: "8.1.0"
     checkout main
     merge jhipster_upgrade tag: "1.3.0"
     commit id: "fix: another cool patch" tag: "1.3.1"
     commit id: "feat: another controller" tag: "1.4.0"
     checkout jhipster_upgrade
-    commit id: "fix(8.1.1): npx jhipster upgrade"
+    commit id: "8.1.0'"
+    checkout main
+    merge jhipster_upgrade type: HIGHLIGHT
+    checkout jhipster_upgrade
+    commit id: "8.1.1"
     checkout main
-    merge jhipster_upgrade tag: "1.4.1"
+    merge jhipster_upgrade tag: "1.5.0"
     commit id: "test: controller"