Switch Dependabot to increase-if-necessary versioning strategy (#278)

To stop the unnecessary version bump PRs like #277.

See:
https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference#versioning-strategy--

Signed-off-by: Ed Morley <501702+edmorley@users.noreply.github.com>

Author: edmorley

.github/dependabot.yml

@@ -4,3 +4,4 @@ updates:
     directory: "/"
     schedule:
       interval: "monthly"
+    versioning-strategy: increase-if-necessary