fix: run compatibility tests on all v4-related branches (#22149)

## Summary

The `testnet_compatibility` and `mainnet_compatibility` tests were only
running on PRs targeting branches matching `^v[0-9]+$` (e.g. `v4`). This
missed three other critical branches:

- `backport-to-v4-staging`
- `v4-next`
- `backport-to-v4-next`

This caused a compatibility break on staging-public (4.2.0-aztecnr.2
bricking nodes due to block header shape changes between 4.1 and 4.2) to
go undetected.

## Fix

Expanded the regex in `yarn-project/bootstrap.sh` to:
```
^(v[0-9]+(-next)?|backport-to-v[0-9]+-(staging|next))$
```

This matches all four required branch patterns while still excluding
`next`, `master`, and other non-release branches.

ClaudeBox log: https://claudebox.work/s/dd9d82043dbc175f?run=1

Author: alexghr

yarn-project/bootstrap.sh

@@ -224,7 +224,7 @@ function test_cmds {
   # Aztec CLI tests
   aztec/bootstrap.sh test_cmds
 
-  if [[ "${TARGET_BRANCH:-}" =~ ^v[0-9]+$ ]]; then
+  if [[ "${TARGET_BRANCH:-}" =~ ^(v[0-9]+(-next)?|backport-to-v[0-9]+-(staging|next))$ ]]; then
     echo "$hash yarn-project/scripts/run_test.sh aztec/src/testnet_compatibility.test.ts"
     echo "$hash yarn-project/scripts/run_test.sh aztec/src/mainnet_compatibility.test.ts"
   fi