Bump spring-boot.version from 2.6.1 to 2.6.4 (#1282)

Bumps `spring-boot.version` from 2.6.1 to 2.6.4.

Updates `spring-boot-starter-web` from 2.6.1 to 2.6.4
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.6.1...v2.6.4)

Updates `spring-boot-starter-undertow` from 2.6.1 to 2.6.4
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.6.1...v2.6.4)

Updates `spring-boot-starter-test` from 2.6.1 to 2.6.4
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.6.1...v2.6.4)

Updates `spring-boot-autoconfigure` from 2.6.1 to 2.6.4
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.6.1...v2.6.4)

Updates `spring-boot-configuration-processor` from 2.6.1 to 2.6.4
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.6.1...v2.6.4)

Updates `spring-boot-starter` from 2.6.1 to 2.6.4
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.6.1...v2.6.4)

---
updated-dependencies:
- dependency-name: org.springframework.boot:spring-boot-starter-web
  dependency-type: direct:development
  update-type: version-update:semver-patch
- dependency-name: org.springframework.boot:spring-boot-starter-undertow
  dependency-type: direct:development
  update-type: version-update:semver-patch
- dependency-name: org.springframework.boot:spring-boot-starter-test
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.springframework.boot:spring-boot-autoconfigure
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.springframework.boot:spring-boot-configuration-processor
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.springframework.boot:spring-boot-starter
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

logbook-parent/pom.xml

@@ -48,7 +48,7 @@
         <spring-security5.version>5.6.0</spring-security5.version>
 
         <spring.version>5.3.14</spring.version>
-        <spring-boot.version>${spring-boot2.version}</spring-boot.version>
+        <spring-boot.version>2.6.4</spring-boot.version>
         <spring-security.version>${spring-security5.version}</spring-security.version>
 
         <junit.version>5.8.2</junit.version>

logbook-spring-boot-webflux-autoconfigure/pom.xml

@@ -18,7 +18,7 @@
             v2 so we have to prevent testing this module against 4-th spring -->
             <properties>
                 <spring.version>${spring5.version}</spring.version>
-                <spring-boot.version>${spring-boot2.version}</spring-boot.version>
+                <spring-boot.version>2.6.4</spring-boot.version>
                 <spring-security.version>${spring-security5.version}</spring-security.version>
             </properties>
         </profile>