Set 10.x milestone on Renovate PRs for main branch (#4821)

.github/renovate.json

diff --git a/.github/renovate.json b/.github/renovate.json
index 73a766f..2422a04 100644
--- a/.github/renovate.json
+++ b/.github/renovate.json
@@ -6,6 +6,7 @@
   "dependencyDashboard": false,
   "enabledManagers": ["gradle", "github-actions"],
   "labels": ["exempt-stale"],
+  "milestone": 1,
   "includePaths": ["gradle/libs.versions.toml", "versions.*", "build.gradle", ".github/workflows/*"],
   "postUpgradeTasks": {
     "commands": [

dev-docs/dependency-upgrades.adoc

diff --git a/dev-docs/dependency-upgrades.adoc b/dev-docs/dependency-upgrades.adoc
index 1bfbabb..a3fe455 100644
--- a/dev-docs/dependency-upgrades.adoc
+++ b/dev-docs/dependency-upgrades.adoc
@@ -71,8 +71,11 @@
 churn from frequently-updated dependencies by delaying merge until a few weeks before a new release. One can also
 choose to change to a less frequent schedule or disable the bot, by editing `renovate.json`.
 
+Renovate PRs against `main` are put in the `10.x` milestone, via the `milestone` option which takes the numeric
+milestone id (see https://api.github.com/repos/apache/solr/milestones). Bump it when `main` moves to a new major.
+
 The `branch_9x` branch has its own `renovate.json` that layers on top of (and inherits from) this one, overriding only
-the schedule and the lockfile/license commands. Grouping rules defined here therefore apply to `branch_9x` too.
+the schedule, the milestone and the lockfile/license commands. Grouping rules defined here therefore apply to `branch_9x` too.
 
 Please note that Solr version prior to 10.X use a versions resolution plugin that uses `versions.lock` instead of
 `libs.version.toml`. Therefore, changes cannot be backported via cherry-pick.