Update dependency org.apache.struts:struts2-core to v6

[dev-mend-for-github-com]

This PR contains the following updates:

Package	Type	Update	Change
org.apache.struts:struts2-core (source)	compile	major	2.3.20 -> 6.1.2

By merging this PR, the issue #4 will be automatically resolved and closed:

Severity	CVSS Score	CVE
Critical	9.3	CVE-2016-1000031
Critical	9.3	CVE-2016-3082
Critical	9.3	CVE-2016-3087
Critical	9.3	CVE-2016-4436
Critical	9.3	CVE-2016-6795
Critical	9.3	CVE-2017-12611
Critical	9.3	CVE-2020-17530
Critical	9.2	CVE-2016-3081
Critical	9.2	CVE-2017-5638
Critical	9.2	CVE-2018-11776
Critical	9.2	CVE-2019-0230
High	8.7	CVE-2015-5209
High	8.7	CVE-2015-5209
High	8.7	CVE-2016-0785
High	8.7	CVE-2016-3092
High	8.7	CVE-2016-4430
High	8.7	CVE-2016-4431
High	8.7	CVE-2016-4433
High	8.7	CVE-2016-4433
High	8.7	CVE-2016-4461
High	8.7	CVE-2017-9787
High	8.7	CVE-2017-9787
High	8.7	CVE-2017-9804
High	8.7	CVE-2017-9804
High	8.7	CVE-2019-0233
High	8.7	CVE-2023-24998
High	7.5	WS-2014-0034
Medium	6.9	CVE-2016-3093
Medium	6.9	CVE-2016-3093
Medium	6.9	CVE-2016-4465
Medium	6.9	CVE-2016-4465
Medium	6.3	CVE-2015-1831
Medium	6.3	CVE-2021-29425
Medium	5.3	CVE-2015-2992
Medium	5.3	CVE-2016-2162
Medium	5.3	CVE-2016-4003

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